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
dictionary attack	A password-guessing attack that tries words, common passwords and other likely values rather than every possible combination.
diff	A Unix and Linux utility that compares two files and reports their differences.
diff patch	A file containing the differences between versions of text or source files, normally in a format that can be applied automatically using a patching utility.
differential backup	A backup containing changes made since the last full backup.
Diffie-Hellman	A public-key key-exchange method allowing two parties to establish shared secret material over an insecure communication channel.
Digital Certificate	An electronic credential used to associate an identity with a public key, commonly used for secure network communications.
digital signature	A cryptographic value generated using a private key that allows others to verify the authenticity and integrity of associated data.
DIMM	Dual In-line Memory Module – a memory module containing multiple memory chips and normally used as system RAM in desktop and server computers.
direct I/O	Input/output performed directly between an application and storage rather than through the normal operating-system filesystem cache.
Direct Message	A private message sent directly from one user account to another user or group of users through an online service.
Directory	A filesystem structure used to organise and contain files and other directories. It is commonly called a folder in graphical interfaces.
directory entry	A filesystem record associating a filename with an inode or other filesystem object.
directory inode	An inode representing a directory within a Unix-like filesystem.
Directory Service	A service that stores and provides information about users, computers, groups and other network resources.
discard	A storage operation, commonly associated with Linux and SSDs, informing a block device that specified sectors are no longer in use.
disk image	A file containing a sector-by-sector or otherwise complete representation of the contents of a storage device, partition or filesystem.
disk quota	A limit imposed on the amount of filesystem storage or number of files that a user or group may consume.
disown	A shell operation that removes a job from the shell's job table so that the shell no longer manages it as one of its jobs.
distributed tracing	A technique for following a request or operation across multiple services or processes by recording correlated trace information.
distributed version control	A version-control model in which each working repository contains its own copy of the project history rather than relying on a single central repository.
dithering	A technique using patterns of neighbouring pixels or values to simulate colours or intensities that cannot be represented directly.
DKIM	DomainKeys Identified Mail – an email authentication mechanism in which outgoing messages are digitally signed so that receiving systems can verify that the message is associated with the claimed domain and has not been altered.
DLL	Dynamic-Link Library – a Windows file containing reusable program code and data that can be shared by multiple applications.
DLL hell	Informal term for problems caused by incompatible versions or conflicting copies of shared libraries, particularly associated historically with Windows software.
DM (social media)	An abbreviation for direct message; a private message sent between users through a social-media or online service.
dm-crypt	A Linux kernel device-mapper target providing transparent block-device encryption.
DMA	Direct Memory Access – a mechanism allowing hardware devices to transfer data directly to or from system memory without requiring the processor to handle every individual transfer.
DMA channel	A logical or hardware resource used by a Direct Memory Access controller to manage data transfers between memory and a device.
DMARC	Domain-based Message Authentication, Reporting and Conformance – an email authentication policy framework allowing domain owners to specify how receiving systems should handle messages that fail SPF or DKIM checks.
dmesg	A Unix and Linux command that displays messages from the kernel message buffer, commonly used for diagnosing hardware and driver problems.
DMZ	Demilitarised Zone – a network segment positioned between or outside trusted internal networks, commonly used to host publicly accessible services.
DNS	Domain Name System – the system that translates human-readable domain names such as example.com into IP addresses.
DNS Cache	A temporary store of DNS lookup results maintained by a computer, browser, router or other system to reduce repeated DNS queries.
DNS propagation	The informal term commonly used to describe the period during which changes to DNS records become visible to different caching resolvers as previous cached information expires.
DNS record	A structured item of information stored in the Domain Name System, such as an address, mail-server, name-server or text record.
DNS Resolver	A service that performs DNS queries and obtains the information required to translate domain names into addresses or other DNS records.
DNSSEC	Domain Name System Security Extensions – extensions to DNS that allow DNS responses to be digitally signed so that their authenticity and integrity can be verified.
Docker	A technology and platform for packaging and running applications in containers.
Docker Container	A running instance of a container image that provides an isolated environment for an application.
Docker Image	A packaged, read-only template containing the files, software and configuration required to create a container.
DoH	DNS over HTTPS – a method of transporting DNS queries and responses over HTTPS, providing encryption and allowing DNS traffic to use the normal web transport.
DOM	Document Object Model – a programming interface representing an HTML or XML document as a tree of objects that software can inspect and modify.
Domain	A named area within the Domain Name System, such as example.org, used to organise Internet names and related information.
Domain Controller	A Windows server that manages authentication and directory information for a domain, typically using Active Directory.
Domain Name	A human-readable name used to identify a website or Internet service, such as example.com.
Domain Name System Record	An entry in DNS that provides information such as an IP address, mail server or other data associated with a domain name.
DoS	Denial-of-Service – an attack intended to make a computer system, network or service unavailable to legitimate users.
DoT	DNS over TLS – a method of transporting DNS queries and responses over an encrypted TLS connection.
dot directory	A Unix/Linux directory whose name begins with a dot and which is normally hidden by default.
dotfile	A Unix/Linux file whose name begins with a dot and which is normally hidden by default in directory listings.
double fork	A Unix daemonisation technique using two successive fork operations to detach a process from its controlling terminal and session.
double free	A programming error in which the same dynamically allocated memory is released more than once.
Download	The process of transferring data from a remote computer or server to a local computer or device.
downstream	A project, distribution or product that incorporates or modifies software obtained from an upstream project.
DPI	Dots Per Inch – a measurement commonly used to describe printer resolution or image scanning resolution.
dpkg	Debian Package – the low-level package-management system used to install, remove and inspect Debian-format software packages.
DPMS	Display Power Management Signalling – standards and mechanisms used to allow a computer to control the power-saving state of a display.
DRAM	Dynamic Random Access Memory – a type of RAM that stores each bit using a capacitor and requires periodic refreshing.
Driver	Software that allows an operating system to communicate with and control a particular hardware device.
Driver Update	A newer version of hardware driver software intended to fix problems, improve compatibility, add features or address security issues.
DRM	Digital Rights Management – technologies intended to control access to or usage of digital content, such as software, music, films or electronic books.
DRY	Don't Repeat Yourself – a software-development principle encouraging information and logic to have a single authoritative representation rather than unnecessary duplication.
DSCP	Differentiated Services Code Point – a field in the IP header used to classify packets for quality-of-service handling.
DSL	Digital Subscriber Line – a family of technologies providing digital data transmission over copper telephone lines.
du	Disk Usage – a Unix and Linux command that reports the amount of storage space occupied by files and directories.
DVCS	Distributed Version Control System – a version-control system in which repositories contain their own project history and can exchange changes with other repositories.
DVI	Digital Visual Interface – a video interface standard developed for connecting computers to digital displays and supporting various combinations of digital and analogue video.
DWARF	A standard debugging data format widely used with Unix-like systems and object formats such as ELF.
dynamic allocation	Allocating memory or another resource during program execution rather than determining its size and location entirely at compile time.
dynamic analysis	The examination of software behaviour while the program is executing, commonly using instrumentation, tracing or debugging tools.
dynamic library	A library whose code can be loaded and shared by programs at run time rather than being copied into each executable.
dynamic linking	Linking in which executable programs use shared libraries that are loaded or resolved at runtime.
dynamic loader	The operating-system component or runtime library responsible for loading shared libraries and resolving dynamically linked program dependencies.
dynamic typing	A programming-language system in which type information is associated with values at run time rather than being fully determined during compilation.
Dyndns	A commonly used abbreviation for Dynamic DNS, referring to services that automatically update DNS records when a network connection receives a changing public IP address.
e2fsck	The filesystem-checking utility used for ext2, ext3 and ext4 filesystems.
EACCES	A Unix error condition indicating that permission was denied for an attempted operation.
EBCDIC	Extended Binary Coded Decimal Interchange Code – a character encoding system developed by IBM and still encountered in some mainframe and legacy computing environments.
ECC	Elliptic Curve Cryptography – a family of public-key cryptographic techniques based on mathematical properties of elliptic curves.
ECC Memory	Error-Correcting Code Memory – computer memory capable of detecting and correcting certain types of data corruption, commonly used in servers and other systems where reliability is important.
ECDH	Elliptic Curve Diffie-Hellman – a key-exchange mechanism based on elliptic-curve cryptography.
echo	A shell command that writes specified text or variable contents to standard output.
Ed25519	A public-key digital-signature scheme based on the Edwards-curve Digital Signature Algorithm using the Curve25519 family of elliptic curves.
Edge Computing	A computing model in which data processing is performed closer to the devices or users generating the data rather than exclusively in a central data centre.
edge server	A server positioned near the users or networks it serves, commonly used by content delivery networks, reverse proxies and distributed applications.
EDID	Extended Display Identification Data – information supplied by a display device describing its capabilities, supported resolutions and timing information.
EDITOR	A Unix and Linux environment variable commonly used by programs to determine which text editor should be launched for interactive editing.
EEPROM	Electrically Erasable Programmable Read-Only Memory – non-volatile memory that can be electrically erased and reprogrammed.
EEPROM programmer	Hardware or software used to read, write or erase electrically programmable non-volatile memory devices.
effective GID	The group ID currently used by a Unix process when the operating system checks group permissions.
effective UID	The user ID currently used by a Unix process when the operating system checks permissions for many operations.
EFI	Extensible Firmware Interface – an earlier firmware interface specification that preceded and contributed to the development of UEFI.
EFI System Partition	A special disk partition containing bootloaders and other files used by UEFI firmware to start operating systems.
EINTR	A Unix error condition indicating that a system call was interrupted by a signal before it completed.
ELF	Executable and Linkable Format – a common binary file format for executable files, object files, shared libraries and core dumps on Unix-like systems.
ELF header	The initial structure in an ELF file describing fundamental properties such as its architecture, file type and entry point.
ELF section	A logical region within an ELF object file containing code, data, symbols or other information.
ELF segment	A loadable or otherwise significant region described by an ELF program header and used by the operating system or loader when handling an executable.
ELF symbol	A named entity in an ELF object or executable file representing functions, variables or other linkable program objects.
Email Alias	An alternative email address that delivers messages to another mailbox or address without requiring a separate mailbox.
Email Header	Information stored with an email message containing details such as sender, recipient, date, routing information and message identifiers.
Embed (social media)	The inclusion of a social-media post, video, image, or other externally hosted content within another webpage or application.
embedded system	A computer system designed as part of a larger device or product, commonly performing a dedicated or specialised function.
eMMC	Embedded MultiMediaCard – an integrated flash-storage package commonly used in embedded computers and mobile devices.
Emoji	A standardised pictorial character used in electronic communication to express an idea, object, emotion, reaction, or other meaning.
encapsulation	The practice of combining data with the operations that manipulate it while restricting direct access to internal implementation details.
Encryption	The process of transforming data into a form that cannot normally be understood without the appropriate key or means of decryption.
endianness	The ordering used to store the individual bytes or components of a multi-byte value in memory.
endianness conversion	The process of changing the byte order of a multi-byte value between little-endian and big-endian representations.
Endorsement (social media)	A public expression of support, approval, recommendation, or approval of a person, product, service, idea, or piece of content.
Engagement	The interactions that users have with online content, such as likes, reactions, replies, comments, shares, reposts, saves, or other forms of participation.
Engagement Bait	Content deliberately designed to encourage users to generate interactions such as likes, comments, shares, or reactions primarily to increase its visibility or ranking.
Engagement Rate	A measurement of the level of audience interaction with content, commonly calculated by comparing interactions with views, impressions, followers, or reach.
ENOENT	A Unix error condition indicating that a requested file, directory or other named object does not exist.
entropy	A measure of unpredictability used in computing and cryptography, particularly important when generating secure random values and cryptographic keys.
enumeration	A programming-language type consisting of a defined set of named constant values.
env	A Unix and Linux command used to display or modify the environment variables available to a process.
environment	The collection of variables, settings and other contextual information made available to a running process by the operating system or parent process.
environment inheritance	The mechanism by which a newly created process receives a copy of the environment variables of its parent process.
Environment Variable	A named value maintained by an operating system or process that can be used to provide configuration information to programs.
EOL	End Of Life – the point at which a software product, release or hardware product is no longer officially maintained or supported.
EOL software	Software that has reached its stated End Of Life and no longer receives normal upstream maintenance or security fixes.
EPERM	A Unix error condition indicating that an operation was not permitted, commonly because the process lacks an appropriate privilege.
epoch	A defined reference point from which elapsed time is measured in a particular system. Unix systems commonly use 1970-01-01 00:00:00 UTC as their epoch.
errno	A C and Unix convention providing a numeric error indication set by certain system calls and library functions when an operation fails.
ESP	EFI System Partition – the disk partition used by UEFI systems to store bootloaders and related firmware-accessible files.
ETag	Entity Tag – an HTTP response-header value used to identify a particular representation of a resource and support cache validation.
Ethernet	A family of wired networking technologies commonly used to connect computers and other devices to local area networks.
Ethernet Frame	A unit of data transmitted over an Ethernet network containing addressing information, payload data and error-detection information.
event loop	A programming mechanism that repeatedly waits for events or I/O activity and dispatches handlers or callbacks for them.
event-driven programming	A programming style in which program behaviour is primarily organised around events and handlers that respond to them.
Exception	An event during program execution indicating that an unusual or error condition has occurred and may require special handling.
exception handling	Programming mechanisms for detecting and responding to exceptional conditions or errors during program execution.
exec	A Unix and Linux shell operation that replaces the current process with another program rather than creating an additional process.
exec family	A group of Unix and C-library functions used to replace the current process image with another program.
Executable	A file containing program instructions that a computer can load and execute.
execve	A Unix system call that replaces the current process image with a new program while retaining the process identifier.
EXIF	Exchangeable Image File Format – metadata embedded in image files by cameras and other devices, commonly containing information such as exposure settings, camera model, orientation and capture date.
EXIF orientation	An EXIF metadata field indicating how an image should be rotated or mirrored for correct display.
exit code	A numerical value returned by a program when it terminates, normally used to indicate success, failure or another result condition.
exit status	A numerical value returned by a Unix or Linux process when it terminates, commonly used by shells and scripts to determine whether a command succeeded.
exit status 0	The conventional Unix/Linux indication that a command or program completed successfully.
exit trap	A shell trap associated with the shell's EXIT event and commonly used to perform cleanup when a script terminates.
exploit	Code, data or a technique that takes advantage of a vulnerability or unintended behaviour in software or hardware.
expression	A combination of values, variables, operators and function calls that can be evaluated to produce a result.
Ext4	A widely used Linux filesystem designed to store files and directories on storage devices.
extended attribute	Additional filesystem metadata stored alongside a file or directory, commonly used for security labels, access-control information and application-specific data.
extent	A contiguous range of filesystem blocks represented as a single allocation record, reducing metadata overhead for large files.
fail-safe	A design principle in which a failure causes a system to enter a state intended to minimise danger or damage.
fail-secure	A design principle in which a failure causes access or operation to default to a more secure state, potentially denying legitimate access.
Failover	The automatic transfer of operations to a backup system or resource when the primary system fails.
failover cluster	A group of systems configured so that a service can be transferred to another node when the currently active node fails.
Fan Account	A social-media account created by a supporter or fan to discuss, celebrate, archive, or otherwise promote a person, group, work, organisation, or other subject.
fan curve	A rule or configuration defining how fan speed should change in response to temperature or other system conditions.
fanotify	A Linux kernel filesystem notification interface providing event and access monitoring capabilities for files and directories.
fast-forward	A Git operation in which a branch reference can simply be advanced to a descendant commit without creating a new merge commit.
FAT	File Allocation Table – a filesystem family originally developed for floppy disks and later widely used on removable storage and other devices.
FAT32	File Allocation Table 32 – a filesystem format widely supported by operating systems and commonly used on removable storage devices.
fault tolerance	The ability of a system to continue operating despite the failure of one or more components.
fcntl	A Unix system call used for operations on file descriptors, including file locking and changing descriptor properties.
FD	File Descriptor – a small integer used by Unix-like operating systems to identify an open I/O resource.
fdatasync	A system call requesting that file data and the filesystem information required to retrieve it be committed to persistent storage, without necessarily synchronising all metadata.
FDD	Floppy Disk Drive – a device used to read and write data on removable magnetic floppy disks.
fdisk	A command-line utility traditionally used on Unix and Linux systems to create and modify disk partition tables.
FidoNet	A store-and-forward computer network originating in the bulletin-board era that allowed independent systems to exchange messages and files using a defined addressing and routing system.
FIDOnet Address	The hierarchical addressing format used by FidoNet to identify nodes and route messages between participating systems.
FIFO	First In, First Out – a method of processing data in which the item that entered a queue first is the first item removed.
file descriptor	A small non-negative integer used by a Unix or Linux process to identify an open file, device, pipe, socket or other input/output resource.
file descriptor 0	The conventional Unix file descriptor for standard input.
file descriptor 1	The conventional Unix file descriptor for standard output.
file descriptor 2	The conventional Unix file descriptor for standard error.
file descriptor leak	A condition in which a process unnecessarily retains open file descriptors, potentially exhausting the process or system limit.
File Extension	The portion of a filename following the final period, commonly used to indicate the type or format of a file.
File Handle	A value used by an operating system or program to identify an open file or other resource.
file hole	An unallocated region within a sparse file that reads as zeroes when accessed.
file locking	A mechanism allowing processes to coordinate access to a file by indicating that a process is using or modifying it.
file mode	The Unix permissions and file-type information associated with a filesystem object.
file offset	The current position within a file associated with an open file description, determining where subsequent read or write operations occur.
File Permission	A rule defining whether a particular user or group is allowed to read, write, execute or otherwise access a file or resource.
File System	The method used by an operating system to organise, store, name and retrieve files on a storage device.
filesystem	A method and associated data structures used by an operating system to organise, store and retrieve files and directories on a storage device or other medium.
filesystem journal	A record maintained by a journaling filesystem describing filesystem changes so that the filesystem can recover more reliably after an interruption or crash.
filesystem label	A human-readable name assigned to a filesystem and stored in its filesystem metadata.
Filesystem Mount	The process of attaching a filesystem to a directory in an operating system so that its files become accessible.
filesystem namespace	A process-specific view of the filesystem hierarchy, particularly relevant to Linux mount namespaces and containers.
filesystem quota	A mechanism limiting the amount of storage or number of filesystem objects that a user, group or other entity may consume.
filesystem snapshot	A point-in-time view of a filesystem or its underlying storage that can preserve access to data as it existed at a particular moment.
filesystem UUID	A Universally Unique Identifier assigned to a filesystem, commonly used by Linux configuration to identify filesystems independently of device names.
filtered port	A network port for which a firewall or other filtering mechanism prevents the scanner from determining whether a service is actually listening.
find	A Unix and Linux command used to search directory trees for files and directories matching specified conditions.
Finger	An early Internet protocol and service used to retrieve information about users on remote computers, such as login status and account information.
fingerprint	A short value derived from a cryptographic key or certificate, commonly used to identify it conveniently and verify that it is the expected key.
Firewall	Hardware or software that monitors and controls network traffic according to defined security rules.
Firmware	Software stored in non-volatile memory that provides low-level control of a hardware device.
firmware update	The process of replacing or modifying the firmware stored in a hardware device to fix problems, add functionality or improve compatibility.
flash memory	A type of non-volatile semiconductor memory that can be electrically erased and reprogrammed, widely used for storage and firmware.
floating point	A method of representing real numbers using a significand and exponent, allowing a wide range of values to be represented with finite precision.
floating-point	A numerical representation capable of representing a wide range of fractional and very large or small values using a significand and exponent.
floating-point exception	An exception or error condition associated with an invalid, undefined or otherwise problematic floating-point operation.
floating-point unit	A processor component or execution unit designed to perform arithmetic operations on floating-point numbers.
flock	A Unix system call and command used to apply advisory locks to files or file descriptors.
Floppy Disk	An older removable magnetic storage medium used to store computer data.
FLOSS	Free/Libre and Open Source Software – a term encompassing software whose licences provide the freedoms associated with both free software and open-source software.
Follower	A user who has chosen to follow another account so that the followed account's content may appear in the follower's feed or activity stream.
Following	The collection of accounts that a social-media user has chosen to follow.
font	A collection of typeface glyphs and associated information used to render text.
For You Page	A personalised social-media content feed intended to present posts or other material that an algorithm predicts may interest the user.
foreground job	A shell job currently associated with the terminal and normally able to receive input from it.
foreground process	A process currently associated with and receiving input from the controlling terminal.
Foreign Key	A database column or combination of columns used to establish a relationship with a key in another table.
Fork	A Unix and Linux operation that creates a new process by duplicating an existing process.
fork bomb	A program or command sequence that repeatedly creates new processes until available process or system resources are exhausted.
fork-exec	The traditional Unix pattern in which a process creates a child using fork and the child then replaces its program image using an exec call.
forked repository	A separate repository created from another repository, commonly used when independent development or changes need to be maintained separately.
forward compatibility	The ability of an older system or implementation to continue functioning with newer data, protocols or systems, usually within defined limits.
forward proxy	A proxy server through which clients send requests to external servers, commonly used for access control, filtering, caching or privacy.
FOSS	Free and Open Source Software – software made available under licences that provide specified freedoms to use, inspect, modify and redistribute the software.
FPS	Frames Per Second – the number of video or animation frames displayed or processed each second.
FPU	Floating-Point Unit – a processor component designed to perform arithmetic operations involving floating-point values.
FQDN	Fully Qualified Domain Name – a complete domain name specifying a host or other resource within the DNS hierarchy, such as server.example.org.
fragmentation	The process of dividing an IP packet into smaller pieces so that it can traverse a network whose MTU is smaller than the original packet.
frame	A unit of data transmitted at the data-link layer, such as an Ethernet frame, containing addressing and control information around a payload.
frame rate	The number of individual frames displayed or processed per second in video or animation, normally expressed in frames per second.
Framebuffer	A region of memory containing the pixel data used to represent an image displayed on a screen.
free	A C standard-library function used to release dynamically allocated memory.
free software	Software distributed under terms intended to guarantee users freedoms to use, study, modify and redistribute the software, as defined by the Free Software movement.
FreeBSD	A free and open-source Unix-like operating system derived from the Berkeley Software Distribution and known for its integrated operating system and networking capabilities.
Freenet	A decentralised peer-to-peer platform designed to allow information to be published and retrieved without relying on a central server.
frontend	The user-facing portion of an application, particularly the interface and client-side software through which users interact with a service.
fsck	File System ChecK – a family of Unix and Linux utilities used to check and, where possible, repair filesystem structures.
fsck pass	The numeric field in an /etc/fstab entry that controls the order in which filesystems are checked by fsck during boot.
fstab	Filesystem Table – the configuration file traditionally used by Unix and Linux systems to define filesystems that can be mounted and their mounting options.
fstab entry	A configuration line in the Unix or Linux filesystem table describing a filesystem, its mount point and associated mounting options.
fsync	A system call requesting that modified file data and associated filesystem information be committed to persistent storage.
FTP	File Transfer Protocol – a network protocol used to transfer files between computers over a network.
FTP Server	A server providing file-transfer services using the File Transfer Protocol.
FTPS	File Transfer Protocol Secure – FTP protected using TLS encryption and authentication.
FTTC	Fibre To The Cabinet – broadband access in which fibre reaches a street cabinet and the remaining connection to the premises uses another medium, traditionally copper.
FTTP	Fibre To The Premises – broadband access in which optical fibre extends all the way to the customer's premises.
full backup	A backup containing a complete copy of the selected data at the time the backup was made.
full duplex	A communication mode allowing both endpoints to transmit and receive simultaneously.
Function	A reusable block of program code designed to perform a particular operation and optionally return a result.
functional programming	A programming paradigm emphasising functions, immutable data and expressions rather than changing shared state and imperative sequences of commands.
FUSE	Filesystem in Userspace – a mechanism allowing filesystems to be implemented by programs running outside the operating-system kernel.
FYP	An abbreviation commonly used for For You Page, a personalised social-media feed containing algorithmically selected content.
gamma	A numerical relationship describing how image or display signal values correspond to perceived or physical brightness.
gamma correction	A transformation applied to image or display values to compensate for nonlinear relationships between stored values and displayed brightness.
garbage collection	Automatic identification and reclamation of memory that is no longer reachable by a program, used by languages such as Java, Python and many others.
garbage collector	The component of a runtime system responsible for automatically identifying and reclaiming memory that is no longer in use.
Gateway	A device or system that provides access between different networks. On a typical home network, the router often acts as the default gateway.
Gateway Address	The IP address of the network device used by a computer to reach destinations outside its local network.
Gbps	Gigabits per second – a unit used to express a data-transfer rate equal to one billion bits per second using decimal units.
gcc	GNU Compiler Collection – a collection of compilers supporting several programming languages, widely used on Linux and other Unix-like systems.
gdb	GNU Debugger – a debugger used to inspect and control the execution of programs and investigate programming errors.
generic programming	A programming technique in which algorithms and data structures are written to operate with a range of data types.
GET	An HTTP method normally used to request a representation of a resource without intending to modify server-side state.
GHz	Gigahertz – a unit of frequency equal to one billion cycles per second, commonly used to describe processor clock speeds.
GID	Group IDentifier – a numerical identifier assigned to a group by a Unix or Linux operating system.
GIF	Graphics Interchange Format – an image format supporting lossless compression, indexed colours and simple animation.
Git	A distributed version control system used to track changes to files, particularly source code, and to coordinate software development.
Git Branch	An independent line of development within a Git repository, allowing changes to be developed separately before being combined.
Git checkout	A Git operation traditionally used to switch branches or restore files from another Git state. Some of its functions are now provided by git switch and git restore.
Git Clone	The process of creating a local copy of an existing Git repository, including its version history.
Git Commit	A recorded set of changes saved to a Git repository, representing a particular state of tracked files.
Git diff	A Git operation that displays differences between versions of files, commits, branches or other Git states.
git fetch	A Git operation that retrieves objects and references from another repository without automatically integrating the changes into the current branch.
Git HEAD	A symbolic reference in Git identifying the commit or branch currently checked out.
Git index	Git's staging area, recording the content that will be included in the next commit.
Git Merge	The Git operation of combining changes from one branch into another branch.
git pull	A Git operation that retrieves changes from a remote repository and then integrates them into the current branch, commonly through merge or rebase.
git push	A Git operation that sends commits and related references from a local repository to a remote repository.
Git rebase	A Git operation that reapplies commits onto a different base commit, producing a revised commit history.
Git remote	A named reference to another Git repository from which changes can be fetched or to which changes can be pushed.
Git repository	A directory containing the data and metadata maintained by Git to record a project's version history.
Git stash	A Git feature that temporarily saves uncommitted changes so that the working tree can be returned to a clean state.
Git tag	A named reference in Git normally used to identify a particular commit, commonly for marking releases.
Git working tree	The files and directories checked out from a Git repository and currently available for editing.
GitHub	An online platform for hosting Git repositories and collaborating on software development.
glob	A shell pattern used to match filenames, commonly using wildcard characters such as * and ?.
glyph	The visual representation of a character or symbol in a particular typeface or font.
GNU	GNU is Not Unix – a free and open-source software project that provides many components commonly used in Unix-like operating systems.
GNU Make	The GNU implementation of the make build automation utility, widely used for compiling and managing Unix and Linux software projects.
GNU toolchain	A collection of GNU development tools commonly including GCC, Binutils, the GNU C Library and associated utilities.
GNU/Linux	A Unix-like operating system environment combining the Linux kernel with software from the GNU project and other sources.
Gopher	A distributed Internet information system that organises documents and services into hierarchical menus. Developed before the Web became dominant, Gopher servers and clients remain available and are sometimes used for retrocomputing and experimental services.
Gopher Client	Software used to browse and retrieve information from Gopher servers.
Gopher Menu	A hierarchical list of resources presented by a Gopher server, allowing users to navigate documents, directories and other services.
Gopher Protocol	An Internet application protocol designed to organise and retrieve information through hierarchical menus and linked resources.
Gopher Server	A server providing documents, directories or other resources through the Gopher protocol.
Gopherhole	An informal term for a site or collection of resources accessible through the Gopher protocol.
GPIO	General-Purpose Input/Output – configurable digital connections on a computer or embedded system that can be controlled by software to read or generate electrical signals.
GPIO header	A connector exposing GPIO pins and other electrical interfaces on a Raspberry Pi or similar single-board computer.
GPIO pull-down	A resistor or configurable input setting that biases a GPIO input towards ground when no external signal is driving it.
GPIO pull-up	A resistor or configurable input setting that biases a GPIO input towards the positive supply voltage when no external signal is driving it.
GPL	GNU General Public License – a widely used free software licence that requires derivative works distributed under the licence to preserve specified freedoms and source-code availability.
GPLv2	Version 2 of the GNU General Public License, a widely used copyleft free-software licence published by the Free Software Foundation.
GPLv3	Version 3 of the GNU General Public License, a copyleft free-software licence addressing areas including patents, technological restrictions and licence compatibility.
GPT	GUID Partition Table – a partitioning standard used with modern computers and storage devices, forming part of the Unified Extensible Firmware Interface specification.
GPU	Graphics Processing Unit – a processor designed to perform graphics calculations and other highly parallel computational tasks.
GRUB	GNU GRand Unified Bootloader – a widely used bootloader capable of loading Linux and other operating systems.
GSM	Global System for Mobile Communications – a standard originally developed for digital cellular telephone networks and widely deployed around the world.
gTLD	Generic Top-Level Domain – a top-level domain not specifically associated with a country or territory, such as .com, .org or .net.
GUI	Graphical User Interface – a user interface that allows interaction with a computer through visual elements such as windows, icons, menus and buttons.
GUI Application	A software application that primarily uses graphical elements such as windows, menus, icons and buttons for user interaction.
GUI Toolkit	A software library or framework providing reusable components for creating graphical user interfaces.
GUID	Globally Unique Identifier – an identifier designed to be sufficiently unique that it can be generated independently without requiring a central authority to allocate each value.
gzip	GNU zip – a widely used lossless data-compression utility and file format commonly found on Unix and Linux systems.
H.264	A widely used video compression standard, also known as Advanced Video Coding, used for applications including television, streaming, video recording and Blu-ray media.
H.265	High Efficiency Video Coding – a video compression standard designed to provide substantially greater compression efficiency than H.264 at comparable image quality.
H.323	An International Telecommunication Union standard for multimedia communication over packet-switched networks, historically used for Internet telephony and video conferencing.
HAL	Hardware Abstraction Layer – a software layer that hides hardware-specific details from higher-level software.
half duplex	A communication mode allowing communication in both directions but not simultaneously.
halt	A command or operating-system action that stops normal system operation, historically without necessarily powering off the hardware.
Handle	A user-selected account name or identifier used to identify and address a person, organisation, or other account on an online service.
Hard Link	A filesystem directory entry that refers directly to an existing file object rather than creating a separate copy of the file data.
hard link count	The number of directory entries referring to a particular inode in a Unix-like filesystem.
hard real-time	A real-time system in which missing a specified timing deadline constitutes a system failure or unacceptable result.
hard reset	A reset that immediately restarts a system or device without allowing normal software shutdown procedures to complete.
hardware clock	A clock maintained by hardware, normally the real-time clock, which can retain the time while a computer is powered off.
Hash	The result of applying a hash function to data, producing a fixed-size value commonly used for integrity checking, indexing and other computing purposes.
hash collision	A situation in which two different inputs produce the same output from a particular hash function.
Hash Function	A mathematical function that converts input data into a fixed-size value, commonly used for indexing, integrity checks and security applications.
Hashtag	A word or phrase preceded by a hash character, used to label, categorise, discover, or group related online content.
HAT	Hardware Attached on Top – a Raspberry Pi expansion board designed to connect to the GPIO header and follow defined mechanical and electrical conventions.
HAT EEPROM	An EEPROM on a Raspberry Pi HAT containing identification and configuration information allowing compatible software to recognise the attached hardware.
HBA	Host Bus Adapter – hardware providing a computer with an interface to a storage or networking bus, such as SCSI or Fibre Channel.
HBA mode	A storage-controller operating mode in which attached disks are presented directly to the operating system rather than being hidden behind hardware RAID abstractions.
HDD	Hard Disk Drive – a storage device that stores data magnetically on rotating disks.
HDMI	High-Definition Multimedia Interface – a digital interface used to transmit digital video and audio between compatible devices.
HDR	High Dynamic Range – a technique or representation allowing a greater range of brightness or luminance than conventional standard-dynamic-range imagery.
head	A Unix and Linux command that displays the beginning of a file or stream of text.
header	The control and addressing information placed before the payload in a network protocol data unit.
header file	A source file containing declarations, definitions or other information intended to be included by a compiler when building source code.
Heap	An area of memory used for dynamically allocated data during program execution.
heap corruption	Damage to the internal structures or allocated objects of a program's heap, commonly caused by programming errors such as out-of-bounds writes or invalid memory operations.
heap fragmentation	A condition in which free memory in a heap becomes divided into many separated blocks, potentially making large allocations difficult.
heartbeat	A periodic signal or message used to indicate that a system, service or network connection is still operating.
heatsink	A component designed to conduct heat away from an electronic device and dissipate it into the surrounding environment.
HEIC	A common filename extension for HEIF image files, particularly associated with photographs produced by Apple devices.
HEIF	High Efficiency Image File Format – a container format for images and image sequences, commonly associated with HEVC-encoded photographs.
here document	A shell construct allowing a block of text in a script to be supplied as standard input to a command.
here string	A shell feature that supplies a short string directly as standard input to a command, commonly using <<< in Bash and related shells.
heredoc	Here Document – a shell syntax for supplying a block of text as standard input to a command or script.
Hexadecimal	A number system using sixteen symbols, 0–9 and A–F, commonly used to represent binary values in a more compact form.
hibernate	A power-management operation that saves system memory to non-volatile storage and powers down the computer, allowing the previous state to be restored later.
High Availability	A design approach intended to keep a computer system or service operational with minimal interruption, often through redundancy and failover.
histogram	A graphical or numerical representation showing the distribution of pixel intensities or colour values in an image.
history	A record maintained by a command shell containing commands previously entered by the user.
HMAC	Hash-based Message Authentication Code – a mechanism combining a cryptographic hash function with a secret key to provide message authentication and integrity checking.
hole	An unallocated region within a sparse file that reads as zeroes but consumes little or no physical storage.
HOME	A Unix and Linux environment variable specifying the path to the current user's home directory.
homograph attack	An attack using visually similar characters from different character sets or scripts to make a malicious identifier appear similar to a legitimate one.
honeynet	A network containing one or more honeypots designed to attract, observe and analyse unauthorised activity.
honeypot	A deliberately exposed system or service designed to attract and observe unauthorised activity, often for security research or intrusion detection.
horizontal privilege escalation	A privilege-escalation scenario in which a user gains access to resources belonging to another user with similar privileges.
Host	A computer or other device connected to a network and capable of communicating using network protocols.
host architecture	The processor architecture on which a program or development tool is currently running.
host key	A cryptographic key used by an SSH server to identify itself to connecting clients and help prevent connections from being silently redirected to an impostor.
Hostname	A human-readable name assigned to a computer or network device.
hot plug	The ability to connect or disconnect hardware while a computer or system remains powered and operating.
hot spare	A standby hardware component that is already connected and ready to take over automatically or quickly when an active component fails.
Hot Swap	The ability to add, remove or replace a hardware component while a computer or system remains powered and operating.
hotfix	A software fix produced and distributed specifically to address an urgent defect or security problem without waiting for a larger scheduled release.
Hotkey	A keyboard key or combination of keys assigned to perform a particular command or action without navigating through menus.
HOTP	HMAC-based One-Time Password – a one-time authentication code generated from a shared secret and a counter.
Hotspot	A location or device providing wireless network access, commonly allowing devices to connect to the Internet.
HSTS	HTTP Strict Transport Security – a web security mechanism allowing a site to instruct compatible browsers to use HTTPS rather than unencrypted HTTP.
HTML	HyperText Markup Language – the standard markup language used to structure content on web pages.
htop	An interactive Unix and Linux process viewer displaying information about running processes and system resource usage.
HTTP	Hypertext Transfer Protocol – the protocol commonly used to transfer web pages and other resources between web browsers and web servers.
HTTP Header	A field transmitted as part of an HTTP request or response containing information controlling or describing the communication, such as content type, caching or authentication information.
HTTP method	A named operation specified by HTTP, such as GET, POST, PUT, PATCH, DELETE or HEAD, indicating the intended action for a request.
HTTP referrer	The URL or related information supplied by a web client indicating the page or resource from which a request originated, subject to browser privacy policies.
HTTP request	A message sent by an HTTP client to a server requesting a resource or asking the server to perform an operation.
HTTP response	A message returned by an HTTP server in response to an HTTP request, normally containing a status code, headers and optionally a response body.
HTTP Status Code	A three-digit code returned by an HTTP server indicating the result or status of a request, such as success, redirection or an error.
HTTP/2	A version of HTTP introducing features including multiplexed streams, header compression and more efficient use of a single connection.
HTTP/3	A version of HTTP that uses QUIC as its transport protocol rather than TCP.
HTTPS	Hypertext Transfer Protocol Secure – HTTP transmitted using encryption, normally through TLS, to protect data exchanged between a browser and web server.
HTTPS certificate	A digital certificate used by an HTTPS server to authenticate its identity and establish cryptographic protection for the connection.
hub	A simple Ethernet networking device that repeats incoming network traffic to all connected ports. Hubs have largely been replaced by switches.
hyperthreading	Intel's name for its implementation of simultaneous multithreading, allowing a physical CPU core to present multiple logical processors to the operating system.
Hypervisor	Software or firmware that creates and manages virtual machines.
I/O	Input/Output – the transfer of data between a computer and external devices, storage, networks or other systems.
I/O Device	A hardware device used to send data to a computer, receive data from it, or perform both functions.
I/O port	A hardware or logical interface through which a processor communicates with an input/output device. In x86 systems the term can also refer specifically to the separate I/O address space.
I/O wait	A measure of time during which a processor is idle because a task is waiting for input/output operations to complete.
I²C	Inter-Integrated Circuit – a serial communication bus commonly used to allow integrated circuits and sensors to communicate over a small number of shared electrical connections.
I2S	Inter-IC Sound – a digital audio interface commonly used to transfer PCM audio between integrated circuits.
ICC profile	A standardised colour profile describing the colour characteristics of a device or colour space for use in colour management.
ICE	Interactive Connectivity Establishment – a framework used to discover and select viable network paths between endpoints, commonly used with WebRTC.
ICMP	Internet Control Message Protocol – a network protocol used for diagnostic, error-reporting and control messages, including those used by ping.
ICMP echo	An ICMP request and response mechanism commonly used by utilities such as ping to test reachability and measure network round-trip time.
ICMPv6	Internet Control Message Protocol version 6 – the ICMP variant used with IPv6, including functions required by IPv6 neighbour and router discovery.
id	A Unix and Linux command that displays information about a user account, including its user ID, primary group ID and supplementary groups.
IDE	Integrated Development Environment – software that combines tools such as a code editor, compiler or interpreter, debugger and project management features.
IDE Controller	A hardware interface used by older computers to connect storage devices using the Integrated Drive Electronics standard.
idempotent	Describing an operation that can be performed multiple times with the same intended result as performing it once.
identify	An ImageMagick command used to display information about an image, such as its format, dimensions, colour depth and metadata.
IDS	Intrusion Detection System – a system that monitors activity for signs of malicious, suspicious or policy-violating behaviour and generates alerts.
IEEE 802.11	The family of IEEE standards defining Wi-Fi wireless local area networking technologies.
IETF	Internet Engineering Task Force – an international standards organisation responsible for developing and maintaining many of the protocols and technical standards used on the Internet.
ifconfig	Interface Configuration – a traditional Unix and Linux command used to display and configure network interfaces. On many modern Linux systems it has been superseded by the ip command.
image metadata	Information associated with an image describing characteristics such as dimensions, camera settings, timestamps, colour information or copyright.
image sequence	A series of related images intended to be processed, displayed or encoded together, such as frames from a video or an animation.
ImageMagick	An open-source suite of command-line tools and libraries for creating, converting, editing and analysing raster images and other image formats.
Imaging	The process of creating an exact or near-exact image of a storage device, partition or filesystem.
IMAP	Internet Message Access Protocol – a protocol used by email clients to access and manage messages stored on a mail server.
IMAP IDLE	An IMAP extension allowing a mail server to notify a connected email client when new messages or other mailbox changes occur without requiring repeated polling.
immutable	Describing an object or value whose contents cannot be changed after it has been created.
imperative programming	A programming paradigm in which software is expressed as a sequence of commands that change program state.
Impression	A recorded instance of content being displayed to a user, whether or not the user interacts with or consciously notices the content.
include file	A source file incorporated into another source file by a language or preprocessor directive.
incremental backup	A backup containing changes made since a previous backup, reducing the amount of data that must be copied compared with a complete backup.
Influencer	A person whose online presence and audience give them the ability to influence the opinions, interests, purchasing decisions, or behaviour of other users.
info page	Documentation in the GNU Info system, providing structured documentation for GNU programs and related software.
inheritance	An object-oriented programming mechanism allowing one class or type to derive behaviour and properties from another.
init	The traditional Unix process with process ID 1, responsible for initial system setup and commonly for adopting orphaned processes. On many modern Linux systems it is provided by systemd.
init process	The initial user-space process traditionally started by the Unix kernel and responsible for starting other system processes. On modern Linux systems this role is commonly performed by systemd.
Init System	The software responsible for starting and managing system processes during the boot process of a Unix-like operating system.
initramfs	Initial RAM Filesystem – a temporary filesystem loaded into memory during Linux startup containing tools and modules needed to locate and mount the real root filesystem.
inode	A filesystem data structure containing metadata about a file or directory, such as ownership, permissions, timestamps and references to the data blocks containing its contents.
inode exhaustion	A condition in which a filesystem has no unused inodes available for creating additional files, even if free disk space remains.
inode number	The numerical identifier assigned to an inode within a Unix filesystem.
inode table	The filesystem structure containing inode records describing files and directories.
inotify	A Linux kernel facility allowing programs to monitor filesystem objects for events such as creation, deletion, modification and renaming.
Input Device	Hardware used to provide data or commands to a computer, such as a keyboard, mouse, scanner or microphone.
input redirection	Shell redirection that causes a command to read its standard input from a file or other specified source.
instance	A particular object created from a class or, more generally, a specific occurrence of a defined type or entity.
instruction cache	A processor cache storing recently used machine instructions.
instruction pipeline	A processor technique in which different stages of multiple instructions are processed concurrently to increase instruction throughput.
instruction pointer	A processor register identifying the address of the current or next instruction to execute.
Instruction Set	The collection of machine-level instructions that a particular processor architecture can execute.
instrumentation	The addition of code, probes or other mechanisms to software so that its behaviour or performance can be measured.
integer overflow	A condition in which an arithmetic result exceeds the range that can be represented by the integer data type being used.
integration test	A test examining the interaction between two or more software components or systems.
inter-process communication	Mechanisms allowing separate processes to exchange data and coordinate their activities.
Interaction	An action performed by a user in relation to online content or another account, such as viewing, liking, replying, sharing, reposting, following, or commenting.
interactive shell	A shell operating interactively with a user, normally accepting commands from a terminal.
interface	A defined set of operations, conventions or properties through which software components or systems interact.
Internet Backbone	The high-capacity network infrastructure and interconnected networks that carry substantial volumes of traffic between major networks and locations.
Internet Relay Chat	A real-time text-based Internet communication system allowing users to communicate in channels and private conversations.
Internet-Draft	A working document published by the IETF to propose or discuss technical specifications before they become, if accepted, an RFC.
interpolation	A mathematical technique used to estimate pixel values when an image is resized, rotated or otherwise transformed.
Interpreter	A program that executes source code or an intermediate representation directly rather than first producing a standalone machine-code executable.
Interrupt	A signal or event that causes a processor to temporarily suspend its current execution so that it can handle another event or task.
interrupt handler	A routine executed in response to a hardware or software interrupt.
IOPS	Input/Output Operations Per Second – a measure of how many individual input/output operations a storage system can perform per second.
IoT	Internet of Things – the concept of connecting physical devices, sensors and other objects to computer networks so that they can exchange data or be controlled remotely.
IP	Internet Protocol – the network protocol responsible for addressing and routing packets between devices and networks.
IP Address	Internet Protocol address – a numerical address assigned to a device or network interface so that it can be identified and reached on an IP network.
IPC	Inter-Process Communication – mechanisms allowing separate processes to exchange data or coordinate their activities.
IPC namespace	A Linux namespace isolating certain System V IPC and POSIX message-queue resources between groups of processes.
IPS	Intrusion Prevention System – a system that monitors network or system activity and can actively block or prevent detected malicious behaviour.
IPsec	Internet Protocol Security – a collection of protocols used to authenticate and encrypt IP communications.
IPv4	Internet Protocol version 4 – the widely used version of IP that uses 32-bit addresses, normally written as four decimal numbers separated by dots.
IPv6	Internet Protocol version 6 – a newer version of IP that uses 128-bit addresses and provides a vastly larger address space than IPv4.
IRC	Internet Relay Chat – a protocol and system historically used for real-time text-based communication in online chat channels.
IRC Bot	A program that connects to an Internet Relay Chat network and automatically performs tasks such as providing information, moderating channels or responding to commands.
IRC Channel	A named discussion area on an Internet Relay Chat network where multiple users can communicate.
IRC Client	Software that connects to an Internet Relay Chat network and allows a user to communicate through channels and private messages.
IRC Network	A collection of interconnected Internet Relay Chat servers providing users with a common messaging environment.
IRC Nickname	The name used to identify a participant on an Internet Relay Chat network.
IRC Operator	A user with elevated privileges on an Internet Relay Chat network or server, normally responsible for administration and moderation.
IRC Server	A server that provides Internet Relay Chat services and connects users with other IRC servers and participants.
IRQ	Interrupt Request – a signal used by hardware to request attention from a processor.
ISDN	Integrated Services Digital Network – a digital telecommunications technology used to carry voice, data and other services over telephone networks.
ISO Image	A file containing a complete representation of the contents and structure of an optical disc.
ISP	Internet Service Provider – a company or organisation that provides access to the Internet.
iteration	One execution of the body of a loop or one step through a sequence of repeated operations.
iterator	An object or mechanism that provides sequential access to the elements of a collection without exposing the collection's underlying representation.
IV	Initialisation Vector – a value used with certain cryptographic modes to ensure that identical plaintext blocks do not necessarily produce identical ciphertext.
IX	Internet Exchange – a point or facility where independent networks exchange traffic directly.
IXP	Internet Exchange Point – a physical location where networks operated by different organisations exchange Internet traffic directly.
jail	An operating-system isolation mechanism restricting a process to a controlled environment. The term is particularly associated with BSD systems.
JANET	Joint Academic Network – the United Kingdom network serving universities, colleges and research institutions, originally established before widespread commercial Internet access.
Java	A general-purpose programming language designed to run on the Java Virtual Machine and widely used for applications, enterprise systems and Android development.
JavaScript	A programming language widely used to add interactive and dynamic behaviour to web pages and web applications.
JBOD	Just a Bunch Of Disks – a collection of storage devices presented individually or combined without the redundancy characteristics of conventional RAID levels.
JIT	Just-In-Time compilation – a technique in which code is compiled into machine instructions during program execution, often to improve performance.
jitter	Variation in the timing or delay of successive network packets or other events, particularly significant for real-time audio and video.
job	A shell-managed process or group of processes, particularly one started interactively and controlled using shell job-control commands.
job control	Shell facilities allowing interactive users to start, stop, suspend and resume processes and manage them as foreground or background jobs.
job specification	A shell notation used to identify a job managed by the shell, commonly using identifiers such as %1.
journal	A structured collection of system and service log messages, commonly managed and queried by systemd-journald on systems using systemd.
journalctl	A command-line utility used to query and display log messages collected by systemd-journald.
journald	The systemd logging service responsible for collecting and storing system and service log messages.
Journaling Filesystem	A filesystem that records intended changes in a journal so that filesystem consistency can be restored more reliably after an unexpected shutdown.
JPEG	Joint Photographic Experts Group – a family of image compression standards widely used for photographs and other continuous-tone images.
JPG	A commonly used filename extension for JPEG (Joint Photographic Experts Group) image files, particularly on systems that historically limited file extensions to three characters.
JSON	JavaScript Object Notation – a lightweight text-based format commonly used to represent and exchange structured data.
JSON API	An API that commonly exchanges structured data using the JSON format.
JTAG	Joint Test Action Group – a standard and interface commonly used for testing, debugging and programming electronic hardware.
Jumbo Frame	An Ethernet frame larger than the traditional maximum Ethernet frame size, commonly used on suitably configured networks to reduce protocol overhead.
Junction	A Windows filesystem feature that redirects access from one directory location to another directory location.
JVM	Java Virtual Machine – the runtime environment that executes Java bytecode and provides platform-independent program execution.
KDE	K Desktop Environment – a free and open-source desktop environment and software ecosystem for Unix-like operating systems, now commonly referred to simply as KDE.
KDF	Key Derivation Function – a cryptographic function used to derive keys from passwords or other secret material.
keep-alive	A mechanism allowing multiple HTTP requests and responses to use the same network connection rather than creating a new connection for each request.
keepalive	A periodic message or mechanism used to detect whether a connection or remote endpoint remains available.