Commonly used computer terminology

This is a brief list of some commonly used terms and expressions used by computer people. It is not an exhaustive list and many of the terms can be found in greater detail in the Wikipedia.

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term	meaning
::1	The IPv6 loopback address corresponding conceptually to the IPv4 address 127.0.0.1.
/bin	A traditional Unix/Linux filesystem hierarchy directory containing essential executable programs. On many modern Linux systems it may be a symbolic link into /usr/bin.
/dev	The Unix/Linux filesystem hierarchy location containing device nodes through which processes access many hardware and virtual devices.
/etc	A Unix/Linux filesystem hierarchy location traditionally containing system-wide configuration files.
/home	A Unix/Linux filesystem hierarchy location conventionally containing users' home directories.
/opt	A Unix/Linux filesystem hierarchy location traditionally used for optional or third-party application software.
/proc	The traditional mount point for the Linux procfs virtual filesystem.
/root	The home directory conventionally used by the Unix/Linux root user.
/run	A Linux filesystem hierarchy location containing volatile run-time data such as process identifiers, sockets and service state.
/sbin	A traditional Unix/Linux filesystem hierarchy directory containing essential system-administration executables. On many modern Linux systems it may be a symbolic link into /usr/bin.
/srv	A Unix/Linux filesystem hierarchy location traditionally intended for data served by system services, such as web or file servers.
/sys	The traditional mount point for the Linux sysfs virtual filesystem.
/tmp	A Unix/Linux filesystem hierarchy location intended for temporary files and data.
/usr	A major Unix/Linux filesystem hierarchy location containing user-space programs, libraries, documentation and other system resources.
/usr/local	The traditional Unix/Linux filesystem hierarchy location for locally installed software that is not managed by the operating system distribution.
/var	A Unix/Linux filesystem hierarchy location containing variable data such as logs, queues, caches and databases.
127.0.0.1	The conventional IPv4 loopback address, referring to the local computer rather than a remote network host.
200 OK	The HTTP status code indicating that a request was successfully processed.
301 Moved Permanently	An HTTP status code indicating that a resource has been permanently assigned a different URL or URI and clients should normally use the new location.
302 Found	An HTTP status code indicating that a resource is temporarily available at another location.
304 Not Modified	An HTTP status code indicating that a cached representation can be used because the requested resource has not changed according to the relevant cache validators.
400 Bad Request	An HTTP status code indicating that the server could not process the request because it was malformed or otherwise invalid.
401 Unauthorized	An HTTP status code indicating that authentication is required or that supplied authentication credentials are not acceptable.
403 Forbidden	An HTTP status code indicating that the server understood the request but refuses to fulfil it.
404 Not Found	An HTTP status code indicating that the requested resource could not be found.
500 Internal Server Error	An HTTP status code indicating that the server encountered an unexpected condition preventing it from fulfilling the request.
502 Bad Gateway	An HTTP status code indicating that a server acting as a gateway or proxy received an invalid response from an upstream server.
503 Service Unavailable	An HTTP status code indicating that the server is currently unable to handle the request, commonly because of overload or maintenance.
504 Gateway Timeout	An HTTP status code indicating that a gateway or proxy did not receive a timely response from an upstream server.
802.1Q	The IEEE standard defining VLAN tagging for Ethernet frames, commonly used to carry multiple VLANs across a single physical link.
A record	A DNS record mapping a hostname to an IPv4 address.
AAAA record	A DNS record mapping a hostname to an IPv6 address.
AArch64	The 64-bit execution architecture defined by ARM for processors implementing the 64-bit ARM instruction set.
ABA problem	A concurrency problem in which a value changes from A to another value and back to A, causing an algorithm to incorrectly assume that nothing changed.
ABI	Application Binary Interface – a specification defining low-level conventions governing how compiled software interacts with the operating system, processor and other compiled software.
ABI break	A change to a binary interface that can cause previously compiled software to stop working correctly with a newer library or system component.
ABI compatibility	The ability of compiled binary components to interoperate without requiring recompilation, according to the applicable binary interface conventions.
abstract class	A programming-language construct representing a class intended to serve as a base for other classes and which may contain operations that are not fully implemented.
abstract data type	A data type defined by the operations that can be performed on it and the behaviour of those operations, rather than by its particular implementation.
access point	A network device providing wireless clients with access to a wired or otherwise connected network.
access port	A switch port normally associated with a single VLAN and typically used to connect an end device.
accumulator	A register or variable used to hold an intermediate result during a sequence of calculations or other operations.
ACID	Atomicity, Consistency, Isolation and Durability – four properties commonly associated with reliable database transactions.
ACL	Access Control List – a list of rules defining which users, groups, computers or network traffic are permitted or denied access to a resource.
ACL Entry	An individual rule within an Access Control List specifying permissions or access restrictions for a particular user, group or other security principal.
ACL mask	In a POSIX ACL, the entry limiting the effective permissions available to named users, named groups and the owning group.
acpi	Advanced Configuration and Power Interface – a standard for operating-system control of power management, hardware configuration and related functions.
ACPI state	A standardised system power state defined by ACPI, such as the working state S0 or various sleep and hibernation states.
AD	Active Directory – Microsoft directory service used to manage users, computers, groups and other resources in Windows-based networks.
ADC	Analogue-to-Digital Converter – an electronic circuit that converts a continuously varying analogue signal into a digital representation that can be processed by a computer or digital device.
address bus	A collection of electrical or logical signals used to specify the memory or I/O address involved in a computer operation.
address space	The set of memory addresses that a processor or process can potentially use.
addressing mode	A rule used by a processor instruction to determine where its operands are located.
Administrator	A user account or role with elevated privileges allowing it to perform system-management tasks that ordinary users cannot perform.
ADSL	Asymmetric Digital Subscriber Line – a DSL technology providing different downstream and upstream data rates, traditionally with greater download capacity.
advisory locking	A file-locking mechanism in which cooperating processes are expected to honour locks, while the operating system does not necessarily prevent access by processes that ignore them.
AEAD	Authenticated Encryption with Associated Data – an encryption mode providing confidentiality and integrity for plaintext while also authenticating additional unencrypted data.
AES	Advanced Encryption Standard – a widely used symmetric encryption algorithm for protecting digital information.
AGP	Accelerated Graphics Port – an older computer expansion interface designed specifically for connecting graphics cards to a motherboard.
AHCI	Advanced Host Controller Interface – a standard interface for SATA host controllers supporting features such as native command queuing and hot plugging.
Air Gap	A security arrangement in which a computer or network is physically or logically isolated from other networks, particularly untrusted networks.
Algorithm	A defined sequence of steps or rules used to solve a problem or perform a computation.
Algorithmic Feed	A social-media feed in which posts or other content are selected and ordered automatically by an algorithm rather than simply being displayed in chronological order.
Algorithmic Recommendation	The automated selection and suggestion of social-media content, accounts, or other material based on algorithms that analyse user behaviour, interests, relationships, or other signals.
Algorithmic Timeline	A social-media timeline whose contents and ordering are determined wholly or partly by an algorithm rather than strictly by publication time.
alias	A shell feature allowing a user to assign an alternative name or shorthand for a command or sequence of commands.
alpha channel	An image channel representing transparency or opacity in addition to the normal colour channels.
alpha release	An early software release intended primarily for development and testing and generally expected to contain incomplete or unstable functionality.
Alt Text	Alternative text describing an image or other visual content, primarily used to make the content accessible to people using screen readers.
ALU	Arithmetic Logic Unit – the part of a CPU that performs arithmetic calculations and logical operations.
anacron	A Unix and Linux scheduling utility designed to run periodic jobs even when the system is not continuously running, making it useful for machines that are switched off regularly.
Anonymous FTP	An FTP service that allows users to connect and download files without providing a normal personal account.
antialiasing	A technique for reducing visible jagged edges and other sampling artefacts in rendered text, graphics and images.
Anycast	A network addressing and routing technique in which multiple geographically distributed systems advertise the same address and traffic is normally routed to a nearby or otherwise preferred instance.
Apache License	A permissive open-source software licence maintained by the Apache Software Foundation and including provisions concerning copyright, patents and redistribution.
API	Application Programming Interface – a defined way for software programs to communicate with each other and exchange data or functionality.
API break	A change to a programming interface that requires existing software using the interface to be modified to remain compatible.
API compatibility	The ability of software to continue using an API without requiring changes when another component or version is introduced.
API endpoint	A specific network-accessible location through which an API provides a particular operation or resource.
API Key	A secret or semi-secret credential used by an application to identify or authenticate itself when accessing an API.
API versioning	The practice of maintaining distinct versions of an API so that changes can be introduced without unexpectedly breaking existing clients.
APIPA	Automatic Private IP Addressing – a feature of Microsoft Windows that automatically assigns an address in the 169.254.0.0/16 range when a computer configured for DHCP cannot obtain an address from a DHCP server.
APM	Advanced Power Management – an older power-management interface that preceded ACPI and is now largely obsolete.
AppArmor	A Linux security system using application profiles to restrict the resources and operations available to individual programs.
append redirection	Shell redirection using operators such as >> to add output to the end of an existing file rather than replacing its contents.
Application	Software designed to perform tasks for a user or another program, such as word processing, browsing or database management.
Application Server	A server that provides an environment for running application software and commonly handles application logic for clients.
apropos	A Unix/Linux command that searches manual-page descriptions for a supplied keyword or phrase.
apt	Advanced Package Tool – the package-management system used by Debian and Debian-derived Linux distributions to install, update and remove software and resolve dependencies.
apt-get	A command-line interface to the Advanced Package Tool used to manage software packages on Debian and Debian-derived Linux systems.
ar	A Unix/Linux utility traditionally used to create and manipulate archives of object files, particularly static libraries.
Archie	An early Internet search service that maintained indexes of files available from anonymous FTP servers. It was developed before the World Wide Web became widespread.
argc	The conventional C-language integer containing the number of command-line arguments supplied to a program.
Argon2	A modern password-hashing and key-derivation algorithm designed to make password guessing expensive in both computation and memory.
argument	A value supplied to a function, command or program to influence its operation.
argument vector	The array of command-line argument strings conventionally passed to a Unix program as argc and argv.
argv	The conventional C-language array containing the command-line arguments supplied to a program.
ARM	A family of processor architectures widely used in mobile devices, embedded systems, single-board computers and servers.
ARM64	A commonly used name for 64-bit ARM architecture, particularly in Linux distributions and software packaging.
ARMv7	A 32-bit ARM architecture generation widely used in embedded systems and earlier Raspberry Pi models.
ARP	Address Resolution Protocol – a protocol used on IPv4 networks to determine the hardware MAC address associated with an IP address.
ARP Cache	A temporary table maintained by a computer containing mappings between IPv4 addresses and MAC addresses.
Array	A data structure containing a collection of elements that can typically be accessed using an index.
artifact	A file or other output produced by a build or software-development process, such as a binary package, test report or generated documentation.
AS	Autonomous System – a collection of IP networks and routers operated under a common routing policy and identified by an Autonomous System Number.
ASCII	American Standard Code for Information Interchange – a character encoding standard originally designed to represent text using 7 bits, including letters, numbers, punctuation and control characters.
ASCII art	Pictures or diagrams created using ordinary text characters, historically common on terminals, bulletin boards and command-line systems.
ASLR	Address Space Layout Randomization – a security technique that randomises the locations of selected program and library memory regions to make certain attacks more difficult.
ASN	Autonomous System Number – a numerical identifier assigned to an autonomous system for use in Internet routing.
Assembler	A program that converts assembly language instructions into machine code or another low-level representation.
Assembly Language	A low-level programming language that represents processor instructions using human-readable mnemonics.
assertion	A statement in software used to verify that a condition expected by the programmer is true, often causing an error or diagnostic if it is not.
asymmetric encryption	Cryptography using a related public and private key pair for operations such as encryption, decryption or digital signatures.
asynchronous I/O	Input/output operations that allow a program to continue performing other work while waiting for the I/O operation to complete.
at	A Unix and Linux command used to schedule a command to run once at a specified future time.
ATA	Advanced Technology Attachment – a family of standards originally used to connect storage devices to computers, later commonly associated with Parallel ATA before Serial ATA became prevalent.
atd	The Unix and Linux daemon responsible for executing commands scheduled using the at command.
atime	Access time – a filesystem timestamp recording when a file was last accessed, subject to filesystem and mount-option behaviour.
Atom	An XML-based web syndication format and associated protocol designed for publishing and subscribing to frequently updated web content.
Atom Feed	An XML-based web-syndication format used to publish updated content such as articles, news and blog posts.
atomic compare-exchange	A synchronisation operation that compares a memory value with an expected value and replaces it if they match, as one atomic operation.
atomic operation	An operation that appears to occur indivisibly from the perspective of other concurrent threads or processes.
atomicity	The property of an operation being treated as indivisible, so that other participants cannot observe an intermediate state.
attack surface	The collection of interfaces, services, software components and other points through which a system could potentially be attacked.
attack vector	The route or technique through which an attacker gains access to a target or causes a security-relevant action.
attestation	A mechanism by which a system provides cryptographically protected evidence about its software or hardware state.
Attribute	A named property or characteristic associated with an object, file, database field, HTML element or other computing entity.
ATX	Advanced Technology eXtended – a widely used specification defining the physical dimensions, mounting arrangements, connectors and power interfaces of computer motherboards and cases.
Audience	The people who can see, receive, or potentially interact with content published by a user, creator, organisation, or account.
Authentication	The process of verifying the identity of a user, device or computer system.
Authentication Server	A server responsible for verifying the identity of users, devices or applications.
Authentication Token	A piece of data used by a system to represent an authenticated session or grant access to a particular service or resource.
Authorisation	The process of determining what an authenticated user or system is permitted to access or do.
authoritative DNS server	A DNS server that holds the authoritative records for one or more DNS zones.
Autoconf	A GNU build-system tool that generates shell scripts capable of configuring software for different Unix-like systems and environments.
Automake	A GNU build-system tool that generates Makefiles from simplified project descriptions.
autonomous system	A collection of IP networks and routers under the control of one organisation or administrative authority and identified for Internet routing purposes.
Autorun	A feature that automatically starts a program or performs an action when removable media or another device is connected.
Autotools	A collection of GNU build tools, including Autoconf and Automake, traditionally used to generate portable build systems for Unix-like software.
AV1	AOMedia Video 1 – an open, royalty-free video compression format developed by the Alliance for Open Media and designed for efficient delivery of high-quality video.
Availability	The degree to which a computer system, service or resource is operational and accessible when required.
AVIF	AV1 Image File Format – an image format based on the AV1 video codec and designed to provide efficient compression with support for features such as high colour depth, transparency and HDR.
awk	A text-processing language and command-line utility commonly used on Unix and Linux systems to search, extract, transform and report information from structured text.
backdoor	A mechanism that provides access to a system or software outside its normal authentication or security controls.
backend	The server-side portion of an application or service responsible for processing requests, accessing data and performing operations not normally carried out by the client.
background job	A shell job running without currently occupying the interactive terminal, allowing the user to continue using the shell.
background process	A process running without occupying the interactive foreground position of the controlling shell.
backport	A software change taken from a newer version and adapted for use in an older supported software release.
backtrace	A representation of the active call stack of a program, particularly useful for diagnosing crashes and debugging.
Backup	A separate copy of data kept so that the original data can be restored if it is lost, damaged, corrupted or accidentally deleted.
backward compatibility	The ability of a newer version of software, hardware or an interface to continue working with data, programs or systems designed for an earlier version.
bad sector	A storage-device sector that cannot reliably store or return data and has been identified as defective.
Bandwidth	The maximum rate at which data can theoretically be transferred over a communication channel or network connection.
Bandwidth Throttling	The deliberate limitation of the data-transfer rate available to a network connection, service or application.
bandwidth-delay product	The amount of data that can be in transit on a network path at one time, calculated from the available bandwidth and round-trip or relevant propagation delay.
banner	Text or protocol information returned by a network service that may identify the service software, version or other characteristics.
banner grabbing	The process of connecting to a network service and examining information it provides in order to identify the service or its characteristics.
base class	In object-oriented programming, a class from which another class inherits properties or behaviour.
base station	A fixed communication station providing connectivity to mobile or wireless devices.
base64	A method of representing binary data as text using a defined set of 64 characters, commonly used when binary data must be transported through text-oriented systems.
Bash	Bourne Again SHell – a widely used command-line shell and scripting language on Linux and Unix-like systems.
BASIC	Beginner's All-purpose Symbolic Instruction Code – a family of programming languages originally designed to make computer programming accessible to beginners.
Batch File	A text file containing a sequence of commands that can be executed automatically by a command interpreter, particularly in Windows.
Baud	A unit describing the number of symbols transmitted per second over a communications channel. A baud is not necessarily equal to one bit per second because a symbol can represent more than one bit.
BBS	Bulletin Board System – a computer system that users could connect to, traditionally over a modem, to exchange messages, download files, play games and access other services.
BBS Door	A program running on a Bulletin Board System that provided an additional service or game to users, such as online games, file utilities or information services.
BBS Sysop	System Operator – the person responsible for operating and maintaining a Bulletin Board System.
bcrypt	A password-hashing algorithm based on the Blowfish key schedule and designed to make password guessing computationally expensive.
BDP	Bandwidth-Delay Product – the amount of data that can be simultaneously in transit over a network path at a given bandwidth and delay.
benchmark	A standardised or repeatable test used to measure and compare the performance of hardware, software or systems.
beta release	A software release made available for wider testing after the main functionality is implemented but before the software is considered fully final.
BGP	Border Gateway Protocol – the routing protocol used to exchange reachability information between autonomous systems on the Internet.
bicubic interpolation	An image-resampling method using a larger neighbourhood and cubic interpolation to produce smoother results than bilinear interpolation.
big endian	A byte-ordering convention in which the most significant byte of a multi-byte value is stored or transmitted first.
big-endian	A byte-ordering convention in which the most significant byte of a multi-byte value is stored at the lowest memory address.
bilinear interpolation	An image-resampling method calculating new pixel values from the weighted average of neighbouring source pixels.
Binary	A number system using only the digits 0 and 1. Computers use binary representation internally for digital data and instructions.
binary compatibility	The ability of existing compiled programs or binary components to continue working with a newer version of a system or library.
binary distribution	A software distribution containing precompiled executable code rather than requiring the recipient to build the software from source.
Binary File	A file containing data stored in a format intended primarily for computer processing rather than direct reading as ordinary text.
binary interface	An interface between compiled software components defined by conventions at the machine-code and object-file level.
binary package	A software package containing precompiled software ready for installation on a compatible system.
bind mount	A Linux mount operation that makes an existing directory or filesystem location accessible at another location in the filesystem hierarchy.
BinkP	A protocol used by FidoNet and related systems to exchange mail and files efficiently between bulletin-board and mail systems over Internet connections.
Binutils	A collection of GNU binary utilities including tools such as the assembler, linker, object-file utilities and symbol inspection programs.
Bio (social media)	A short description on a social-media profile in which a user or organisation introduces themselves or provides information about the account.
BIOS	Basic Input/Output System – firmware traditionally used to initialise computer hardware and start the operating system when a computer is powered on.
BIOS interrupt	A software interface provided by the BIOS through which software can request certain low-level firmware services on traditional x86 systems.
birth time	A filesystem timestamp representing when a file was created, where the operating system and filesystem support recording it.
Bit	The smallest commonly used unit of digital information, capable of representing either a 0 or a 1.
Bit Depth	The number of bits used to represent each sample or value, commonly affecting the possible range of colours in images or amplitude levels in digital audio.
bit field	A group of one or more bits within a value used to represent a particular flag, state or small integer.
bit shift	An operation that moves the bits of a binary value left or right, usually inserting or discarding bits according to the operation used.
bitmask	A binary value used to select, clear, set or test particular bits within another value.
Bitrate	The amount of digital data transmitted or processed per unit of time, normally expressed in bits per second.
bitstream	A sequence of bits representing encoded data, such as compressed audio, video or other information.
BitTorrent	A peer-to-peer file distribution protocol in which files are divided into pieces that can be downloaded from multiple participating computers and simultaneously uploaded to other participants.
BitTorrent Client	Software that participates in the BitTorrent peer-to-peer protocol by downloading and uploading pieces of files.
BitTorrent Tracker	A service that helps BitTorrent clients discover other peers participating in the distribution of particular content.
bitwise AND	A bitwise operation producing a one in each bit position where both operands contain a one.
bitwise NOT	A bitwise operation that inverts every bit of its operand.
bitwise operation	An operation performed directly on the individual bits of a binary value, such as AND, OR, XOR or bit shifting.
bitwise OR	A bitwise operation producing a one in each bit position where at least one operand contains a one.
bitwise XOR	Exclusive OR – a bitwise operation producing a one where the corresponding bits of the operands differ.
block	A fixed-size unit of data or storage used by filesystems, block devices and other computer systems.
Block (social media)	A function that prevents another social-media account from contacting, viewing, following, or otherwise interacting with the blocking account, subject to the particular platform rules.
block device	A device file representing hardware or virtual storage that can be accessed in blocks, such as a hard disk, solid-state drive or partition.
block group	A subdivision of a filesystem containing filesystem metadata and data blocks, particularly associated with filesystems such as ext2, ext3 and ext4.
block size	The size of the fixed units into which data is divided for storage, transfer or processing.
blocking I/O	I/O in which a calling process or thread waits rather than continuing execution until the operation can proceed or complete.
Bluetooth	A short-range wireless technology used to exchange data between compatible devices.
BNC	Bayonet Neill–Concelman – a bayonet-style connector commonly used for coaxial cables in radio, video, test equipment and networking applications.
bonding	A Linux networking mechanism allowing multiple physical network interfaces to be combined into one logical interface for purposes such as redundancy or increased throughput.
Bookmark (social media)	A function that allows a user to save a social-media post or other content for later access without necessarily publicly interacting with it.
Boolean	A data type or logical value that has two possible states, commonly represented as true and false.
Boost (social media)	A social-media action that republishes or amplifies another user's content so that it is shown to a wider audience.
Boot	The process of starting a computer and loading its operating system into memory.
boot parameter	A parameter supplied to the kernel during system boot to configure its behaviour or hardware support.
boot ROM	Read-only or non-volatile firmware containing the initial code executed when a computer or embedded device starts.
Boot Sector	A sector of a storage device containing information or executable code used during the process of booting a computer.
Bootloader	A small program responsible for starting the operating system or another larger program during the computer boot process.
bootstrap	The process by which a computer starts execution of increasingly capable software, beginning with low-level firmware or boot code.
Bootstrap Protocol	An early protocol used by computers to obtain network configuration information during startup, preceding widespread use of DHCP.
Bottleneck	A component or operation that limits the overall performance or throughput of a computer system.
Bourne Shell	The original Unix shell written by Stephen Bourne and commonly known as sh. It established many of the shell scripting conventions still used on Unix and Linux systems.
BPS	Bits Per Second – a unit describing the rate at which digital information is transmitted or processed.
brace expansion	A shell feature that generates multiple strings from a pattern containing brace expressions such as {a,b,c}.
branch	A named line of development in a version-control system, allowing changes to be developed independently from another line.
branch prediction	A processor technique for predicting the outcome or target of a conditional branch so that instruction execution can continue without waiting for the actual result.
breakpoint	A debugging facility that temporarily stops program execution at a specified location so that the program state can be examined.
bridge	A device or software function that connects network segments at the data-link layer and forwards frames between them.
Broadcast	A networking method in which data is sent to all devices within a defined network broadcast domain.
broadcast domain	The collection of network devices that can receive a particular layer-two broadcast transmission.
broadcast storm	A condition in which excessive broadcast traffic overwhelms a network, potentially consuming substantial bandwidth and processing resources.
Browser	Software used to access and display websites and other content on the World Wide Web.
brute-force attack	An attack that systematically tries many possible passwords, keys or other values until an acceptable one is found.
BSD	Berkeley Software Distribution – a family of Unix-derived operating systems originating at the University of California, Berkeley, including FreeBSD, OpenBSD and NetBSD.
BSD licence	A family of permissive open-source software licences originating from the Berkeley Software Distribution and commonly imposing relatively few restrictions on reuse and redistribution.
BSSID	Basic Service Set Identifier – an identifier used in Wi-Fi networking to identify a particular wireless access point or wireless network interface.
btrfs	B-tree File System – a Linux filesystem supporting features including snapshots, checksums, subvolumes, compression and copy-on-write storage.
Buffer	An area of memory used to temporarily hold data while it is being transferred between devices, processes or different parts of a system.
buffer cache	Memory used by an operating system to cache filesystem or block-device data in order to reduce physical storage operations.
buffer overflow	A programming error in which data is written beyond the bounds of a buffer, potentially corrupting memory or enabling security vulnerabilities.
buffer underflow	A condition in which an operation attempts to read or otherwise use data beyond the valid lower boundary of a buffer or data structure.
buffer underrun	A condition in which a process requires data from a buffer but the buffer does not contain data quickly enough to keep the operation supplied.
Bug	An error or defect in software that causes it to behave incorrectly or unexpectedly.
build system	Software responsible for automating compilation, linking, testing, packaging and related steps required to produce software.
bulletin board	An online system or discussion area where users can post and read messages. The term predates the modern web and remains associated with BBS systems and forums.
Bus	A communication pathway used to transfer data, addresses or control signals between components of a computer.
bus error	A hardware-related exception raised when a program attempts an invalid memory access or performs an operation that the processor or memory subsystem cannot support.
Bus Mastering	A technique allowing a hardware device to transfer data directly to or from memory without requiring the CPU to handle every transfer.
Bus Speed	The rate at which data or signals are transferred across a computer bus.
Bus Topology	A network topology in which multiple devices share a common communication medium or backbone.
Byte	A unit of digital information consisting of 8 bits. A byte can represent 256 different values.
Bytecode	An intermediate form of program code designed to be executed by a virtual machine or interpreter rather than directly by the physical processor.
C	A general-purpose programming language widely used for operating systems, embedded software, utilities and performance-critical applications.
C#	A general-purpose programming language developed by Microsoft and commonly used for applications, web services and the .NET platform.
C++	A general-purpose programming language based on C that adds features including object-oriented and generic programming.
CA	Certificate Authority – an organisation or system trusted to issue and digitally sign certificates used to establish cryptographic identities.
Cache	A high-speed area of memory or storage used to keep frequently accessed data so that it can be retrieved more quickly.
cache hit	A memory-access event in which requested data is found in the relevant CPU cache.
cache line	The fixed-size block of memory transferred between a CPU cache and a lower level of the memory hierarchy.
cache miss	A memory-access event in which requested data is not present in the relevant CPU cache and must be obtained from a lower level of the memory hierarchy.
cache-control	An HTTP response-header mechanism allowing a server to specify caching behaviour for clients and intermediary caches.
CAD	Computer-Aided Design – the use of computer software to create, modify and analyse technical drawings and three-dimensional models.
call stack	A data structure maintained by a program to keep track of active function or procedure calls and their associated local data.
callback	A function or other callable object supplied to software so that it can be invoked later when a specified event or operation occurs.
calling convention	A set of rules defining how functions receive arguments, return values and preserve registers when compiled programs communicate at the binary level.
calloc	A C standard-library function that allocates memory for an array and initialises the allocated bytes to zero.
canonical mode	A Unix terminal input mode in which input is normally made available to a program a line at a time after line-editing processing.
capabilities	The individual privileged operations that can be granted to Linux processes through the kernel capability mechanism.
capability	A fine-grained Linux privilege mechanism allowing selected privileged operations without granting a process full superuser privileges.
capability bounding set	A Linux security mechanism limiting the capabilities that a process and its descendants may possess.
capability-based security	A security model in which access rights are represented by protected references or capabilities rather than relying solely on global identity and permission checks.
CAPTCHA	A test intended to distinguish human users from automated programs, often used to reduce abuse of websites and online services.
CAS	Compare-And-Swap – an atomic operation that conditionally replaces a value when it matches an expected value.
case insensitive	Describing a system in which uppercase and lowercase forms of letters are treated as equivalent for a particular operation.
case sensitive	Describing a system in which uppercase and lowercase letters are treated as different characters or values.
cast	A programming-language operation that explicitly converts a value from one data type to another.
cat	A Unix and Linux command used to read files and write their contents to standard output, and commonly used to combine multiple files.
Cat5e	Category 5e – a twisted-pair Ethernet cabling category commonly supporting Gigabit Ethernet over appropriate installation distances.
Cat6	Category 6 – a twisted-pair Ethernet cabling category designed for higher-frequency signalling and supporting Gigabit Ethernet and, under suitable conditions, higher speeds.
Cat6a	Category 6a – an enhanced twisted-pair cabling category designed to support 10 Gigabit Ethernet over standard structured-cabling distances.
ccTLD	Country Code Top-Level Domain – a top-level domain assigned to a country or territory, such as .uk or .fr.
cd	Change Directory – a shell command used to change the current working directory.
CD-R	Compact Disc Recordable – a recordable optical disc that can normally be written to once.
CD-ROM	Compact Disc Read-Only Memory – an optical disc format used to distribute data that is normally read rather than written.
CD-RW	Compact Disc ReWritable – an optical disc format that allows data to be erased and rewritten.
CDN	Content Delivery Network – a distributed network of servers that delivers web content from locations closer to users to improve performance and reduce latency.
certificate	A digitally signed data structure used to associate an identity, such as a domain name, with a public cryptographic key.
certificate authority	An organisation or service that issues and digitally signs certificates binding identities to public keys.
certificate chain	A sequence of digital certificates linking an end-entity certificate to a trusted certificate authority through one or more intermediate authorities.
certificate revocation	The process of invalidating a digital certificate before its normal expiration date.
CGI	Common Gateway Interface – a standard allowing a web server to execute external programs and use their output to generate dynamic web content.
CGI Script	A program executed by a web server through the Common Gateway Interface to generate dynamic responses to web requests.
cgroup	Control Group – a Linux kernel mechanism for organising processes into groups and controlling or accounting for their use of resources such as CPU and memory.
cgroup v2	The current unified Linux control-group hierarchy providing resource control and accounting for groups of processes.
ChaCha20	A stream cipher designed by Daniel J. Bernstein and widely used with Poly1305 for authenticated encryption.
character device	A device file representing hardware or a virtual device that communicates as a stream of characters or bytes rather than as addressable blocks.
character encoding	A defined mapping between characters and their binary representations.
character set	A defined collection of characters that a system or encoding is capable of representing.
Checksum	A calculated value derived from data and used to detect accidental changes or corruption.
checksum file	A file containing checksums or cryptographic hashes for one or more files, allowing their integrity to be verified later.
chgrp	Change Group – a Unix and Linux command used to change the group ownership of a file or directory.
chmod	Change Mode – a Unix and Linux command used to change the permission bits associated with files and directories.
chown	Change Owner – a Unix and Linux command used to change the owner and, optionally, group associated with a file or directory.
chroot	Change Root – a Unix and Linux operation that changes the apparent root directory for a process and its descendants.
chroot jail	An environment created using chroot to restrict a process to a selected directory hierarchy, although chroot alone is not a complete security boundary.
CI	Continuous Integration – the practice of automatically building and testing software as changes are integrated into a shared codebase.
CI/CD	Continuous Integration and Continuous Delivery/Deployment – practices and automation used to build, test and deliver software changes regularly.
CIDR	Classless Inter-Domain Routing – a method of representing IP network prefixes using a network address followed by the number of significant bits, such as 192.0.2.0/24.
cipher	An algorithm used to encrypt and decrypt data.
ciphertext	Data produced by encrypting plaintext.
CIX	Compulink Information eXchange – a British online service and conferencing system that provided discussion forums, email and other services before and during the early growth of the public Internet.
CLI	Command Line Interface – a text-based interface through which commands can be entered to control a computer or run programs.
CLI Shell	A command-line shell that accepts commands from a user or script and passes them to the operating system or other programs.
Client	A computer or software application that requests and uses services or resources provided by another system, commonly called a server.
Client-Server	A computing model in which client programs request services or resources from server programs.
clone	The process of creating a local copy of a version-control repository, including its history.
closed port	A network port at which no service is listening, causing connection attempts to be rejected or otherwise reported as unavailable.
closure	A function together with the surrounding lexical environment needed to access variables from the scope in which the function was created.
Cloud Computing	The use of remote computers and servers accessed over a network, usually the Internet, to provide computing, storage, software or other services.
Cluster	A group of computers or storage resources working together as a unit, often to improve availability, performance or scalability.
CMake	A cross-platform build-system generator that produces build files for tools such as Make and Ninja.
CMOS	Complementary Metal-Oxide-Semiconductor – a semiconductor technology commonly associated with low-power circuitry. The term is also traditionally used for the small amount of motherboard memory storing firmware configuration settings.
CMYK	Cyan, Magenta, Yellow and Key (black) – a colour model primarily used for colour printing.
CNAME	Canonical Name – a DNS record that specifies an alias for another domain name.
Coaxial Cable	A cable consisting of a central conductor surrounded by insulation and a conductive shield, commonly used for radio-frequency, television, networking and measurement applications.
COBOL	Common Business-Oriented Language – a programming language originally designed for business data processing and still encountered in some long-established commercial systems.
code path	A possible sequence of instructions or branches through a program during execution.
code point	A numerical value assigned to a character or other element in a character set such as Unicode.
code review	The process of examining source-code changes to identify defects, security issues, maintainability problems or other concerns before integration.
Codec	A technology or algorithm used to encode and decode digital data, particularly audio or video.
cold backup	A backup created while the system or application being backed up is stopped or otherwise not actively changing its data.
cold boot	Starting a computer from a powered-off state.
cold spare	A spare component kept available for replacement but not actively powered or participating in the system.
collision domain	A network segment in which simultaneous transmissions can potentially interfere with one another. Modern switched full-duplex Ethernet largely eliminates collisions on individual links.
colour channel	One component of a digital colour representation, such as red, green or blue in an RGB image.
Colour Depth	The number of bits used to represent the colour of each pixel in a digital image. Greater colour depth allows a larger number of possible colours or intensity levels.
Colour Management	The use of standards, profiles and software to maintain consistent colour representation between cameras, scanners, monitors, printers and other imaging devices.
colour profile	Data describing how numerical colour values correspond to actual colours in a particular device or colour space.
Colour Space	A defined mathematical model describing how colours are represented numerically. Examples include RGB and CMYK colour spaces.
COM Port	A traditional Windows designation for a serial communications interface, commonly used for physical or virtual serial ports.
Command	An instruction entered by a user or supplied by a program to request that a computer perform a particular operation.
command line	A textual interface in which a user enters commands and arguments for a shell or other command interpreter to execute.
Command Prompt	A Windows command-line interface that allows users to execute commands and programs using text input.
command substitution	A shell feature that executes a command and substitutes its output into another command or expression.
command-line argument	A value supplied to a program when it is started, normally appearing after the program name on a command line.
commit	A recorded set of changes in a version-control repository, forming part of its history.
commit hash	A cryptographic identifier derived from the contents and metadata of a version-control commit.
Community Note	A user-contributed explanatory or contextual note attached to social-media content, typically intended to provide additional context or identify potentially misleading information.
compare-and-swap	An atomic processor or synchronisation operation that changes a value only if it still matches an expected value.
compile-time	The stage during which source code is translated, checked or otherwise processed before execution.
Compiler	A program that translates source code written in a programming language into machine code or another lower-level form that can be executed by a computer.
compiler flag	An option supplied to a compiler controlling aspects of compilation such as optimisation, warnings, language standards or generated code.
compiler optimisation	Techniques used by a compiler to transform source code into executable code that is smaller, faster or otherwise more efficient while preserving its required behaviour.
compiler toolchain	The collection of software tools used to transform source code into executable programs, typically including a compiler, assembler, linker and supporting utilities.
compiler warning	A diagnostic produced by a compiler indicating a construct that may be incorrect, suspicious or undesirable even though compilation can continue.
composite	An ImageMagick operation or command used to combine one image with another using operations such as overlaying or masking.
Compression	The process of reducing the amount of storage space or transmission bandwidth required by data.
Computer Vision	The use of computers to analyse, interpret and extract information from images or video.
concurrency	The ability of multiple tasks to make progress during overlapping periods of time, whether or not they are executing simultaneously.
condition variable	A synchronisation primitive allowing threads to wait until a particular condition becomes true.
conditional	A programming construct that selects which statements to execute depending on whether a specified condition is true or false.
conditional compilation	A compilation technique in which sections of source code are included or excluded according to predefined conditions.
configure script	A generated shell script commonly used by Autoconf-based software projects to detect the local system environment and prepare a build.
connection timeout	A period after which an application or network service gives up waiting for a connection to be established or for an expected network response.
constant	A named value whose value is intended not to change during the relevant execution or scope of a program.
constructor	A special operation used to initialise a newly created object.
Container	An isolated software environment used to package an application together with its dependencies while sharing the host operating system kernel.
container format	A file format capable of holding encoded media and associated metadata, allowing different codecs or data streams to be packaged together.
Containerisation	A method of packaging software and its dependencies into isolated execution environments that share an operating-system kernel.
Content Creator	A person or organisation that regularly produces and publishes original or curated digital content for an online audience.
Content Moderation	The process of reviewing, filtering, restricting, removing, or otherwise managing user-generated content according to the rules of an online service or community.
Content Warning	A notice indicating that content may contain material that some users could find disturbing, offensive, upsetting, or otherwise undesirable.
Context Menu	A menu displayed by a graphical user interface in response to an action such as right-clicking, normally containing commands relevant to the selected object or location.
context switch	The process by which an operating system saves the execution state of one process or thread and restores the state of another.
continuous delivery	A software-development practice in which software is maintained in a state where it can be released reliably on demand.
continuous deployment	A software-development practice in which changes that pass automated checks are automatically deployed to production.
continuous integration	A software-development practice in which changes are frequently integrated and automatically built and tested.
control group	A group of processes managed together by an operating system for purposes such as resource accounting, limits and scheduling.
controlling terminal	The terminal associated with a Unix process session and used for terminal input, job-control signals and related functions.
controlling TTY	The terminal device associated with a process session and used for interactive input, output and job control.
convert	An older ImageMagick command-line tool used for image conversion and processing. In modern ImageMagick versions, magick is generally preferred.
Cookie	A small piece of data stored by a website in a web browser, commonly used to remember preferences, sessions or other information about a visitor.
Cookie Session	A mechanism in which a web browser stores a cookie containing information used by a website to associate requests with a particular user session.
cooperative multitasking	A multitasking model in which a running process or task is expected to voluntarily yield control to allow others to execute.
copy-on-write	A technique in which data is initially shared and a private copy is made only when one participant needs to modify it.
copy-on-write snapshot	A snapshot mechanism that initially shares storage with the original data and records changed blocks separately as modifications occur.
copyleft	A licensing principle in which users are granted freedoms to use, modify and redistribute software while conditions require certain redistributed modified versions to preserve those freedoms.
core affinity	The association of a process or thread with one or more specific CPU cores on which it is permitted to execute.
Core Dump	A file containing information about the memory and state of a program at the time it terminated abnormally, commonly used for debugging.
core file	A file containing information about a process at the time it terminated abnormally, commonly used for debugging crashes.
core pattern	The Linux kernel setting controlling how and where core dump files are generated.
coredumpctl	A systemd utility used to inspect and retrieve information about core dumps collected by systemd-coredump.
CORS	Cross-Origin Resource Sharing – a browser security mechanism that controls whether web pages can request resources from a different origin.
CoW	Copy-on-Write – a technique delaying duplication of shared data until a modification requires a private copy.
cp	Copy – a Unix and Linux command used to copy files and directories.
CP/M	Control Program for Microcomputers – an early operating system widely used on microcomputers before the widespread adoption of MS-DOS.
CPE	Common Platform Enumeration – a structured naming scheme used to identify hardware, operating systems, applications and other technology products.
CPU	Central Processing Unit – the main processor in a computer that executes program instructions and performs calculations.
CPU affinity	A setting controlling which processor cores a process or thread is permitted to execute on.
CPU Cache	Small, high-speed memory located within or close to a processor that stores frequently accessed instructions and data.
CPU Core	An individual processing unit within a CPU capable of executing instructions.
CPU profiler	A profiling tool designed to determine how processor time is distributed among functions, threads or other parts of a program.
CPU register	A small, high-speed storage location inside a processor used directly by machine instructions.
CRC	Cyclic Redundancy Check – an error-detection technique used to determine whether data has been accidentally corrupted during storage or transmission.
Creator (social media)	A person who produces and publishes content for an audience through one or more social-media or online platforms.
credential stuffing	An attack using usernames and passwords obtained from one service to attempt authentication against other services.
credentials	Information used to establish or verify identity or access rights, such as passwords, cryptographic keys or authentication tokens.
critical section	A portion of code that accesses shared resources and must be protected so that conflicting concurrent operations cannot execute simultaneously.
CRL	Certificate Revocation List – a digitally signed list of certificates that a certificate authority has revoked.
cron	A Unix and Linux system facility for automatically running commands or scripts at scheduled times.
cron job	A command or script configured to run automatically according to a cron schedule.
cron.d	A directory containing system-wide cron schedule files on many Unix and Linux systems.
crontab	A table containing scheduled commands for cron, together with the times and conditions under which those commands should be run.
cross compilation	The process of building software on one system for execution on a different target architecture or platform.
cross compiler	A compiler that runs on one type of computer or operating system but produces executable code for another platform.
cross-compilation	The process of compiling software on one architecture or operating system for execution on another architecture or operating system.
cross-compiler	A compiler that runs on one platform while generating executable code for a different target platform.
cross-platform	Describing software designed to operate on more than one operating system or hardware platform.
Cross-posting	Publishing the same or substantially similar content to multiple online services, communities, or social-media accounts.
CRT	Cathode-Ray Tube – a display technology using an electron beam to illuminate phosphors on the inside surface of a glass screen.
CRUD	Create, Read, Update and Delete – the four basic operations commonly performed on data stored in a database or application.
cryptographic hash	A hash function designed to make it computationally difficult to find two different inputs with the same hash and to recover an input from its hash.
Cryptography	The study and application of techniques for protecting information through encryption, authentication, integrity mechanisms and related methods.
cryptsetup	A Linux utility used to configure and manage encrypted block devices, including LUKS volumes.
CSLIP	Compressed Serial Line Internet Protocol – an extension of SLIP that reduces the overhead of transmitting TCP/IP headers over serial connections.
CSP	Content Security Policy – an HTTP-based security mechanism allowing a website to restrict which types and sources of content a browser may load or execute.
CSPRNG	Cryptographically Secure Pseudo-Random Number Generator – a pseudo-random number generator designed to produce values that are difficult for an attacker to predict.
CSRF	Cross-Site Request Forgery – an attack in which a victim's authenticated browser is induced to send an unintended request to a web application.
CSS	Cascading Style Sheets – a language used to control the presentation, layout, colours, fonts and appearance of HTML documents.
ctime	Change time – a Unix filesystem timestamp recording when inode metadata was last changed. It is not the file creation time.
cut	A Unix and Linux command used to extract selected sections, fields or characters from each line of input.
CVE	Common Vulnerabilities and Exposures – a standardised system for identifying publicly disclosed cybersecurity vulnerabilities using identifiers such as CVE-2026-12345.
CVSS	Common Vulnerability Scoring System – a standardised framework for assessing the severity of cybersecurity vulnerabilities.
CWE	Common Weakness Enumeration – a catalogue of common classes of software and hardware weaknesses that can lead to security vulnerabilities.
D-Bus	Desktop Bus – an inter-process communication system commonly used by Linux desktop environments and system services to exchange messages.
DAC	Digital-to-Analogue Converter – an electronic circuit that converts digital numerical values into a continuously varying analogue signal.
Daemon	A background process that runs without direct user interaction and commonly provides a service to an operating system or network.
Daemon Process	A background process that normally operates without direct user interaction and provides a service to the system or other applications.
daemonisation	The process of turning a Unix program into a background daemon, traditionally involving steps such as forking, detaching from the terminal and establishing suitable standard streams.
data cache	A processor cache storing recently accessed data.
Data Centre	A facility containing computer systems, networking equipment, storage and supporting infrastructure used to operate computing services.
Data Mining	The process of analysing large collections of data to discover patterns, relationships and useful information.
data structure	A defined way of organising and storing data so that it can be accessed and manipulated efficiently.
Data Type	A classification defining the kind of data a variable, database field or other storage location can contain and how that data is interpreted.
Data Warehouse	A centralised repository designed to store and analyse large quantities of data, often collected from multiple sources.
Database	An organised collection of data that can be stored, searched, updated and managed by computer software.
Database Index	A data structure maintained by a database to speed up searches and retrieval of rows, usually at the cost of additional storage and update overhead.
Datagram	A self-contained unit of data transmitted across a network without requiring a dedicated connection.
DBMS	Database Management System – software used to create, store, organise, query and manage databases.
DCC	Direct Client-to-Client – an Internet Relay Chat mechanism allowing IRC users to establish direct connections for activities such as file transfers and private conversations.
dd	A Unix and Linux command used for copying and converting data between files, devices and other input or output sources. It is commonly used for tasks such as creating disk images.
DDNS	Dynamic Domain Name System – a service that automatically updates DNS information when the public IP address associated with a network connection changes.
DDoS	Distributed Denial-of-Service – an attack in which many systems send traffic or requests to a target in an attempt to make a service unavailable.
DDR	Double Data Rate – a memory technology that transfers data on both edges of a clock signal, effectively allowing two data transfers per clock cycle.
dead code	Program code that cannot be reached or executed under the relevant conditions, often indicating obsolete or unnecessary logic.
Deadlock	A situation in which two or more processes or threads are unable to continue because each is waiting for a resource held by another.
DEB	A Debian software package file containing software, metadata and installation information for systems using the Debian package-management system.
DEB package	A software package in the Debian package format, containing files and metadata required to install software on Debian and compatible systems.
Debian	A free and open-source operating system distribution known for its large software repository, community development and use as a foundation for other Linux distributions.
Debian package repository	A collection of Debian software packages and associated metadata made available through a network or local storage location for use by Debian package-management tools.
debouncing	The process of filtering rapid unwanted transitions produced by a mechanical switch when it is pressed or released.
debug build	A software build configured to retain additional debugging information and commonly use reduced optimisation to make debugging easier.
debug level	A logging level intended for detailed diagnostic information useful during development or troubleshooting.
debug symbols	Information embedded in or associated with compiled software allowing debuggers to relate machine code to source files, variables and functions.
debugger	Software that allows a programmer to inspect and control a running program, including setting breakpoints, examining variables and stepping through instructions.
Debugging	The process of finding, diagnosing and correcting errors or unexpected behaviour in software.
declaration	A programming-language construct that tells the compiler or interpreter about a name, type, function or other program element.
Default Gateway	The network device to which a computer sends traffic intended for destinations outside its local network.
defence in depth	A security strategy using multiple independent layers of protection so that failure of one control does not necessarily compromise the entire system.
DELETE	An HTTP method used to request deletion of a specified resource.
DEP	Data Execution Prevention – a security mechanism preventing selected memory regions from being executed as program code.
Dependency	A software component, library, package or other resource that a program requires in order to operate.
dependency confusion	A software supply-chain attack in which a package manager is tricked into retrieving a malicious package with a name that takes precedence over a legitimate internal dependency.
dependency graph	A representation showing dependencies between software packages, libraries, modules or other components.
dependency hell	A situation in which conflicting, incompatible or difficult-to-manage software dependencies make installation or maintenance unusually complicated.
dependency tree	A hierarchical representation of the direct and indirect dependencies required by a software package or application.
deprecated	Describing a software feature or interface that remains available but is discouraged in favour of a newer alternative.
deprecation	The designation of a software feature, interface or function as discouraged or scheduled for eventual removal while it may remain available for compatibility.
dereference	The operation of accessing the object or value referred to by a pointer or similar reference.
destructor	A special operation used to perform cleanup when an object is destroyed or its lifetime ends.
detached HEAD	A Git state in which HEAD refers directly to a commit rather than to a branch name.
devfs	A device filesystem mechanism providing filesystem representations of devices. On modern Linux systems, device management is primarily handled through devtmpfs and related mechanisms.
Device Driver	Software that allows an operating system to communicate with and control a particular hardware device.
device mapper	A Linux kernel subsystem providing a framework for creating virtual block devices from mappings and transformations of underlying storage.
device node	A special filesystem entry representing a device and allowing applications to communicate with it through the normal Unix file interface.
devtmpfs	A Linux virtual filesystem containing device nodes maintained by the kernel and commonly mounted at /dev.
df	Disk Free – a Unix and Linux command that reports the amount of used and available space on mounted filesystems.
DH	Diffie-Hellman – a cryptographic key-exchange method.
DHCP	Dynamic Host Configuration Protocol – a network protocol that automatically provides devices with configuration information such as IP addresses, subnet masks and gateways.
DHCP client	A computer or network device that obtains network configuration from a DHCP server.
DHCP Lease	A period during which a DHCP server assigns an IP address and other network configuration information to a client.
DHCP Reservation	A DHCP configuration that assigns a particular IP address to a device based on its network interface identifier, such as its MAC address.
DHCP Server	A network service that automatically assigns IP addresses and other network configuration information to clients.
DHT	Distributed Hash Table – a distributed data structure allowing a collection of networked nodes to locate information without relying on a single central database.
Dial-up	A method of connecting a computer to a network or Internet service using a telephone line and a modem.
Dial-up Networking	The use of a modem and telephone connection to establish a computer network connection, commonly used for Internet access before broadband became widespread.
DICOM	Digital Imaging and Communications in Medicine – an international standard for storing, transmitting and exchanging medical imaging information and associated data.