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
Kerberos	A network authentication system using trusted third parties and cryptographic tickets to provide secure authentication across a network.
Kermit	A communications protocol and family of software tools designed for reliable file transfer and terminal communication, particularly over serial and low-speed connections.
Kernel	The core component of an operating system that manages hardware resources and provides fundamental services to software.
kernel command line	The set of parameters passed to the Linux kernel during boot, commonly visible through /proc/cmdline.
kernel mode	The processor execution mode in which the operating-system kernel can perform privileged operations and access protected resources.
Kernel Module	A piece of code that can be loaded into or removed from an operating-system kernel to add functionality such as hardware support.
kernel panic	A fatal Linux kernel error from which the kernel cannot safely continue, normally causing the system to halt or restart.
kernel parameter	A configurable value controlling some aspect of operating-system kernel behaviour.
Kernel Space	The privileged memory and execution environment in which an operating-system kernel operates with direct access to protected system resources.
kerning	The adjustment of spacing between particular pairs or combinations of characters to improve the visual appearance of text.
Key	In cryptography, a value used by an algorithm to encrypt, decrypt, authenticate or otherwise transform data. In databases, a key can also identify or relate records.
key derivation function	A cryptographic function used to derive one or more keys from a password or other secret input, normally incorporating a salt and computational cost.
key exchange	A cryptographic procedure allowing parties to establish shared secret key material over a communication channel.
Key Pair	A pair of related cryptographic keys consisting of a public key and a private key used in asymmetric cryptography.
KiB	Kibibyte – a unit of digital information equal to 1,024 bytes.
kill	A Unix and Linux command used to send a signal to a process. Despite its name, it can send many different signals and is not limited to terminating processes.
killall	A Unix and Linux command used to send a signal to processes selected by name rather than by individual process ID.
KISS	Keep It Simple, Stupid – an engineering principle favouring simple designs over unnecessary complexity.
known_hosts	An SSH configuration file containing information about remote hosts that have previously been encountered and their associated host keys.
KVM	Kernel-based Virtual Machine – a Linux kernel technology that allows the system to operate as a hypervisor and run virtual machines.
L1 cache	Level 1 CPU cache – the smallest and typically fastest cache located closest to a processor core.
L2 cache	Level 2 CPU cache – a CPU cache generally larger and slower than L1 but faster than main memory.
L3 cache	Level 3 CPU cache – a larger, generally slower cache commonly shared by multiple processor cores.
LACP	Link Aggregation Control Protocol – a protocol used to negotiate and manage the bundling of multiple physical network links into a logical link.
lambda	An anonymous function expression that can be created and passed around as a value in programming languages that support functional-style constructs.
LAN	Local Area Network – a computer network covering a relatively small geographical area such as a home, office or building.
LAN Adapter	A hardware device or network interface that allows a computer to connect to a local area network.
LANG	An environment variable commonly used to specify the default locale and character-language settings for a Unix or Linux process.
Last-Modified	An HTTP response-header indicating when a resource was last modified, allowing clients and caches to perform conditional requests.
Latency	The time delay between initiating an operation and receiving the corresponding response or result.
LBA	Logical Block Addressing – a method of identifying storage blocks using numerical logical block addresses rather than physical geometry.
ld	The Unix/Linux linker supplied as part of GNU Binutils or another toolchain, responsible for combining object files and libraries into executable or shared binary objects.
LD_LIBRARY_PATH	An environment variable used on Unix-like systems to specify additional directories in which the dynamic linker should search for shared libraries.
LD_PRELOAD	A Unix/Linux environment variable allowing specified shared libraries to be loaded before other libraries, commonly used for testing, debugging or controlled function interposition.
ld.so	The Linux dynamic linker and loader responsible for loading shared libraries and resolving dynamic dependencies of executable programs.
LDAP	Lightweight Directory Access Protocol – a protocol used to access and manage directory information such as users, groups and computers.
LDAP client	Software allowing a computer or application to access directory information using the Lightweight Directory Access Protocol.
ldconfig	A Unix and Linux utility that manages the shared-library cache and symbolic links used by the dynamic linker.
ldd	A Unix and Linux command used to display the shared libraries required by an executable program.
least privilege	The security principle that a user, process or service should have only the permissions necessary to perform its required tasks.
Leech	A participant in a peer-to-peer file-sharing system that downloads data while contributing relatively little data for upload to other participants.
Leeching	The practice of downloading data from a peer-to-peer network while contributing little or no data back to other participants.
lexer	A component of a compiler or parser that converts a sequence of characters into tokens such as identifiers, keywords, operators and literals.
LGPL	GNU Lesser General Public License – a free-software licence designed to permit linking with software under other licences while retaining copyleft requirements for the covered library itself.
Library	A collection of reusable software code or resources that can be used by other programs.
LibreOffice	A free and open-source office productivity suite providing applications for word processing, spreadsheets, presentations, databases and other office tasks.
Libtool	A GNU build-system tool designed to simplify building and using shared libraries across Unix-like systems.
licence	A legal permission defining how software, data or other intellectual property may be used, modified, copied or distributed.
Like	A social-media interaction indicating that a user approves of, appreciates, or otherwise positively reacts to a piece of content.
limits.conf	A Linux configuration file used by the PAM limits mechanism to define resource limits for users and sessions.
line discipline	Kernel functionality controlling how characters are processed between a terminal device and an application, including functions such as canonical input and echoing.
link aggregation	The combining of multiple physical network links into a single logical connection to provide increased capacity and/or redundancy.
link count	The number of directory entries or hard links referring to a Unix filesystem inode.
link-time	The stage of software construction during which object files and libraries are combined into a linked executable or other binary.
linked list	A data structure consisting of elements, called nodes, in which each node contains data and one or more references to other nodes.
Linker	A program that combines object files and libraries to produce an executable program or another final program image.
linker script	A file containing instructions controlling how a linker combines object files and arranges sections and addresses in the resulting executable or binary.
linking	The process of combining compiled object files and libraries into an executable or another binary output.
lint	A static-analysis tool or the process of using one to identify probable programming errors, suspicious constructs, style violations or other source-code problems.
Linux	A family of open-source operating systems based on the Linux kernel, commonly used on servers, desktop computers, embedded devices and other systems.
Linux distribution	A complete operating-system package built around the Linux kernel together with software, libraries, package-management tools and configuration supplied by a particular project or organisation.
listen	The state in which a network service waits for incoming connection requests on a specified address and port.
literal	A value written directly in source code, such as a number, character or quoted string.
little endian	A byte-ordering convention in which the least significant byte of a multi-byte value is stored or transmitted first.
little-endian	A byte-ordering convention in which the least significant byte of a multi-byte value is stored at the lowest memory address.
livelock	A concurrency problem in which processes or threads remain active but repeatedly change state without making useful progress.
Livestream	A live transmission of audio, video, or other content over the Internet while it is being produced or broadcast.
ln	Link – a Unix and Linux command used to create hard links and symbolic links to files.
Load	The amount of work currently being performed by a computer system or one of its components.
Load Average	A measure commonly used on Unix-like systems to indicate the amount of computational work waiting to be processed over specified time intervals.
Load Balancer	A device or software system that distributes network requests or workloads across multiple servers or other resources.
locale	A set of regional and language conventions used by software and operating systems for such things as character classification, sorting, number formatting and date representation.
localhost	A hostname that refers to the local computer or device. It normally resolves to a loopback IP address.
locate	A Unix and Linux command used to search a pre-built database of filenames, generally providing faster results than searching the filesystem directly.
lock	A synchronisation mechanism used to control access to a shared resource by multiple threads or processes.
lock file	A file used to indicate that a resource is currently in use or that a particular operation is already in progress.
lock-free	Describing a concurrent algorithm in which system-wide progress is guaranteed even if individual threads may be delayed.
Log File	A file containing records of events, activities, errors or other information generated by a computer system or application.
log level	A classification indicating the importance or severity of a log message, such as debug, informational, warning or error.
logging	The practice of recording events, messages, errors or other information produced by software while it operates.
logic bomb	Malicious code designed to execute a harmful action when a particular condition or event occurs.
Logical Core	A processor execution context presented to the operating system, which may correspond to a physical CPU core or to a hardware thread.
logical volume	In LVM, a virtual block device allocated from a volume group and normally used like a conventional disk partition.
login shell	A shell started as part of a user login session and normally performing login-specific initialisation.
logrotate	A Unix and Linux utility used to automatically rotate, compress, archive and remove old log files according to configured policies.
Loop	A programming construct that repeatedly executes a block of instructions while a specified condition remains true or until a termination condition is reached.
loop device	A Linux block-device interface allowing a regular file to be accessed as though it were a block device, commonly used with disk images.
Loopback	A networking mechanism that allows a computer to send network traffic back to itself, commonly using 127.0.0.1 for IPv4.
lossless compression	Compression that reduces the size of data without losing information, allowing the original data to be reconstructed exactly.
lossy compression	Compression that reduces data size by discarding information judged less important, meaning the original data cannot be reconstructed exactly.
ls	List – a Unix and Linux command used to display information about files and directories.
LSM	Linux Security Module – a kernel framework providing hooks through which security systems such as SELinux and AppArmor implement additional access-control policies.
lsof	List Open Files – a Unix and Linux utility that reports which processes have files, devices, sockets or other filesystem resources open.
ltrace	A Unix and Linux diagnostic utility that traces calls made by a program to shared libraries.
LTS	Long-Term Support – a software release or support policy in which a particular version is maintained with updates and security fixes for an extended period.
LUKS	Linux Unified Key Setup – a standard for implementing encrypted storage volumes on Linux systems.
LUKS2	The second major version of the Linux Unified Key Setup format, providing improvements to metadata handling and cryptographic configuration.
LVM	Logical Volume Manager – a Linux storage-management system that provides flexible management of physical storage and logical volumes.
LXC	Linux Containers – operating-system-level virtualisation technology providing isolated Linux environments that share the host kernel.
Lynx	A text-based web browser originally developed for Unix systems and still useful for accessing websites from text-only environments.
LZMA	Lempel-Ziv-Markov chain Algorithm – a data compression algorithm known for achieving high compression ratios and used by formats such as 7z and XZ.
M.2	A small form-factor specification for internally mounted computer expansion devices, commonly used for solid-state storage and wireless adapters.
MAC	Media Access Control – a layer of networking concerned with addressing and controlling access to a shared network medium. MAC is also commonly used as shorthand for a MAC address.
MAC Address	Media Access Control address – a hardware-level network interface identifier normally assigned to a network adapter.
MAC Learning	A switch function that records which MAC addresses are associated with particular network ports so that Ethernet traffic can be forwarded efficiently.
Machine Code	Binary instructions directly executable by a particular processor architecture.
machine instruction	A binary-coded operation directly understood and executed by a processor.
macro	A named rule that causes source code or other text to be expanded or substituted during preprocessing or another translation stage.
Magick	The modern command-line interface provided by ImageMagick for performing image-processing operations.
Magnet Link	A URI identifying content in a peer-to-peer network by information such as a cryptographic hash rather than directly specifying the location of a file.
Magnet URI	A Uniform Resource Identifier containing information used by peer-to-peer software to identify content, commonly through a cryptographic hash.
Mail Server	A server responsible for sending, receiving, storing or delivering email messages.
Mailing List	A system that distributes email messages sent to a particular address to a group of subscribed recipients.
Mainframe	A large computer system designed to process substantial volumes of data and support many users or applications, commonly used by financial, governmental and other large organisations.
maintainer	A person or organisation responsible for maintaining a software package, project, repository or other technical resource.
major number	The device-number component identifying the kernel driver associated with a Unix device node.
major version	The first component of a semantic version number, normally incremented when incompatible changes are introduced.
Majordomo	Mailing-list management software used to automate tasks such as subscribing and unsubscribing users and distributing messages to list members.
make	A build automation utility that determines which files need to be rebuilt and executes the commands required to update them.
make install	A conventional build-system target used to install software built from source into the configured destination directories.
Makefile	A file containing rules and dependency information used by the make build automation tool.
malloc	A C standard-library function that dynamically allocates a specified amount of memory.
Malware	Malicious software designed to damage systems, disrupt operations, steal information or gain unauthorised access.
man	Manual – a Unix and Linux command used to display the system manual pages documenting commands, functions, configuration files and other system interfaces.
man page	A manual page documenting a command, function, configuration file, file format or other feature of a Unix or Linux system.
man-in-the-middle	An attack in which an attacker positions themselves between communicating parties and intercepts or potentially alters their communications.
mandatory locking	A file-locking mechanism in which the operating system can prevent conflicting access to a locked file.
MANPATH	An environment variable specifying directories in which Unix/Linux manual pages may be found.
MBR	Master Boot Record – the first sector of a traditionally partitioned storage device containing boot information and a partition table.
MDA	Mail Delivery Agent – software responsible for delivering received messages into a user's mailbox or local mail storage.
measured boot	A boot process in which components are cryptographically measured and the measurements are recorded for later verification or attestation.
Media Access Control	A data-link networking mechanism concerned with addressing network interfaces and controlling access to a shared communications medium.
Meme	A piece of digital content, often combining an image, video, phrase, or other media with a recurring idea or joke, that is copied, adapted, and circulated online.
Memory	Electronic storage used by a computer to hold data and program instructions for processing. The term commonly refers to RAM.
memory alignment	The arrangement of data in memory so that objects begin at addresses meeting the alignment requirements of the processor or data type.
memory allocator	Software responsible for managing dynamically allocated memory and supplying portions of it to programs when requested.
memory barrier	A processor or compiler operation that constrains the ordering of memory accesses as observed by different processors, threads or devices.
Memory Bus	The communication pathway connecting a processor or memory controller with system memory.
memory corruption	A condition in which a program accidentally modifies memory belonging to itself or another data structure in an unintended manner.
Memory Leak	A programming error in which memory that is no longer required remains allocated and cannot be effectively reused.
Memory Management	The operating-system functions responsible for allocating, tracking, protecting and releasing computer memory.
memory model	The rules defining how memory operations become visible to processors, threads or other parts of a computing system.
memory page	A fixed-size block of virtual or physical memory used by an operating system's memory-management system.
memory pressure	A condition in which available physical memory is low enough that the operating system must reclaim memory, evict caches or use swap more aggressively.
memory profiler	A profiling tool designed to measure memory allocation, usage, leaks or related behaviour in a program.
memory-mapped file	A file whose contents are mapped into a process's virtual address space, allowing the program to access file data through memory operations.
memory-mapped I/O	A hardware architecture in which device registers are assigned addresses within the processor's normal memory address space.
Mention	A reference to another user or account within online content, usually using an account identifier that creates a link or notification for the mentioned account.
merge	The process of combining changes from two development histories into a single history.
merge conflict	A condition in which a version-control system cannot automatically reconcile conflicting changes made to the same part of a file or related project data.
Meson	A modern build-system framework designed to provide fast and portable software builds, commonly generating files for Ninja.
message queue	An inter-process communication mechanism in which processes exchange discrete messages through a managed queue.
Metadata	Data that describes or provides information about other data, such as file size, creation time, author or database structure.
metapackage	A package containing little or no software itself but depending on other packages so that a collection of related software can be installed together.
method	A function associated with an object or class in object-oriented programming.
method overriding	A mechanism by which a derived class provides its own implementation of a method inherited from a base class.
metric	A numerical measurement used to describe the state, performance or behaviour of a system.
MFA	Multi-Factor Authentication – an authentication method requiring two or more independent factors to verify a user identity.
MHz	Megahertz – a unit of frequency equal to one million cycles per second.
microarchitecture	The internal implementation of a processor architecture, including execution units, caches, pipelines and other hardware structures.
microbenchmark	A benchmark designed to measure the performance of a small operation, function or narrowly defined system component.
microcode	Low-level processor control information used by some CPUs to implement or control the execution of machine instructions.
Microcontroller	A compact integrated circuit containing a processor, memory and peripherals designed to control electronic devices or embedded systems.
Microprocessor	An integrated circuit containing the central processing unit of a computer or other electronic system.
microSD	A small form-factor variant of the Secure Digital removable flash-memory card standard, widely used in embedded devices.
Middleware	Software that provides common services or communication functions between applications, operating systems and other software components.
MIDI	Musical Instrument Digital Interface – a standard for communicating musical performance and control information between electronic musical instruments, computers and related devices.
MIME	Multipurpose Internet Mail Extensions – standards used to identify and represent different types of Internet content, including email attachments and web resources.
MIME type	A label identifying the format or media type of data, such as text/html, image/jpeg or application/json.
minor number	The device-number component used by a device driver to distinguish individual devices or functions handled by that driver.
minor version	The second component of a semantic version number, normally incremented when backward-compatible functionality is added.
MIT licence	A short permissive open-source software licence allowing broad use, modification and redistribution subject primarily to preservation of the copyright and licence notice.
MITM	Man-In-The-Middle – an attack in which an attacker intercepts communications between two parties while attempting to remain undetected.
mkdir	Make Directory – a Unix and Linux command used to create directories.
mkfs	Make Filesystem – a family of Unix and Linux commands used to create a filesystem on a storage device or partition.
mknod	Make Node – a Unix and Linux command used to create special filesystem entries such as device files.
mmap	A Unix system call that maps files or other objects into a process's virtual address space.
MMIO	Memory-Mapped Input/Output – accessing hardware device registers through addresses in the processor's memory address space.
MMU	Memory Management Unit – hardware responsible for translating virtual addresses to physical addresses and enforcing various memory-access permissions.
mock object	A test object designed to simulate the behaviour of another component so that software can be tested in isolation.
Modem	A device that converts digital data into signals suitable for transmission over a communications medium and converts received signals back into digital data.
Modem Pool	A collection of modems maintained by an organisation or service provider to allow multiple users to establish dial-up connections.
Moderator	A person or account responsible for enforcing the rules of an online community, discussion area, or other user-generated content service.
module parameter	A value supplied when loading a kernel module to configure its behaviour.
monorepo	A software-development repository containing multiple projects, applications or components within a single version-control repository.
montage	An ImageMagick utility that combines multiple images into a single composite image, commonly arranged as a tiled montage.
MOO	MUD, Object-Oriented – a type of programmable text-based multi-user virtual environment based on object-oriented concepts.
MOTD	Message Of The Day – a message displayed to users when they connect to a computer system, network service or online community.
Motherboard	The main circuit board of a computer onto which the processor, memory, storage interfaces and other components are connected.
Motherboard BIOS	Firmware on a motherboard that performs hardware initialisation and traditionally provides the first stage of the boot process.
mount	A Unix and Linux command used to attach a filesystem to a directory in the system filesystem hierarchy.
mount namespace	A Linux kernel feature providing a process or group of processes with its own view of mounted filesystems.
mount point	A directory in a Unix or Linux filesystem hierarchy at which another filesystem is attached and made accessible.
MPEG	Moving Picture Experts Group – an organisation and family of standards concerned with compression and representation of digital audio and video.
MQTT	Message Queuing Telemetry Transport – a lightweight publish-and-subscribe messaging protocol commonly used for machine-to-machine communication and Internet of Things applications.
MTA	Mail Transfer Agent – software responsible for transferring email between mail servers using SMTP or related mechanisms.
MTBF	Mean Time Between Failures – a statistical measure of the average operating time between failures of a repairable system or component.
mtime	Modification time – a filesystem timestamp recording when the contents of a file were last modified.
MTTR	Mean Time To Repair – a measure of the average time required to restore a failed system or component to working order.
MTU	Maximum Transmission Unit – the largest packet payload that can normally be transmitted across a particular network link without fragmentation at that layer.
MTU discovery	The process of determining the largest packet size that can traverse a network path without requiring fragmentation.
MUA	Mail User Agent – software used by a person to compose, read and manage email messages.
MUD	Multi-User Dungeon – a text-based multiplayer virtual world in which users interact with other players and the environment through commands or text descriptions.
Multiboot	A computer configuration in which multiple operating systems are installed and a bootloader allows the user to select which one to start.
Multicast	A networking method in which data is sent to a selected group of receiving devices rather than to a single destination or every device.
Multicast DNS	A DNS-based mechanism that allows devices on a local network to discover services and resolve hostnames without a conventional DNS server.
multiple inheritance	An object-oriented programming mechanism allowing a class to inherit directly from more than one base class.
Multitasking	The ability of an operating system to manage multiple processes or applications so that they can appear to run concurrently.
Multithreading	The use of multiple threads within a program so that different sequences of instructions can execute concurrently.
MUSH	Multi-User Shared Hallucination – a type of text-based multi-user virtual environment derived from MUD software and commonly allowing users to create and program parts of the environment.
mutable	Describing an object or value whose contents can be changed after it has been created.
Mute (social media)	A function that hides or suppresses another account's content or notifications from a user without necessarily unfollowing or blocking that account.
mutex	Mutual Exclusion – a synchronisation mechanism used to ensure that only one thread or process at a time can access a protected resource.
mutex lock	An operation that acquires exclusive ownership of a mutex so that the calling thread can safely enter a protected critical section.
mv	Move – a Unix and Linux command used to move or rename files and directories.
MX	Mail Exchange – a DNS record specifying the mail servers responsible for accepting email for a domain.
MX Record	Mail Exchange record – a DNS record specifying the mail servers responsible for receiving email for a domain.
named pipe	A FIFO inter-process communication mechanism represented by a filesystem name and usable by unrelated processes.
namespace	A Linux kernel mechanism providing isolated views of particular system resources, used extensively by containers.
NAND flash	A type of flash memory optimised for high-density storage and commonly used in SSDs, memory cards and USB storage devices.
nano	A small, easy-to-use text editor commonly available on Unix and Linux systems and operated primarily from the command line.
NAS	Network Attached Storage – a storage system connected to a network that provides files or other storage services to multiple clients.
NAT	Network Address Translation – a technique that translates IP addresses between different address spaces, commonly allowing multiple private devices to share one public IPv4 address.
NAT Traversal	Techniques used to allow network connections to operate across devices performing Network Address Translation.
NDP	Neighbour Discovery Protocol – an IPv6 protocol used for functions including neighbour discovery, address resolution and router discovery.
nearest-neighbour	An image-resampling method that assigns each new pixel the value of the nearest source pixel. It is fast but can produce visibly blocky results.
NetBEUI	NetBIOS Extended User Interface – a small, non-routable networking protocol historically used by Microsoft Windows and LAN Manager systems.
NetBIOS	Network Basic Input/Output System – an interface and set of networking services historically used by computers on local networks, particularly in conjunction with Microsoft networking.
NetBSD	A free and open-source Unix-like operating system derived from BSD and known for supporting a very wide range of computer architectures.
netmask	Another term for a subnet mask, particularly common in traditional Unix and Linux networking configuration.
network interface	A hardware or software component through which a computer sends and receives network traffic.
network namespace	A Linux isolation mechanism providing a process or group of processes with an independent view of network interfaces, routing tables, ports and related networking resources.
network segment	A portion of a network sharing a particular physical, logical or broadcast domain.
News Feed	A continuously updated stream of posts, articles, updates, or other content presented to a user by an online service.
Newsgroup	A named discussion area in the Usenet system where users can post and read messages on a particular subject.
Newsreader	Software used to read, organise and post messages to Usenet newsgroups.
NFS	Network File System – a protocol allowing filesystems on one computer to be accessed over a network by another computer.
NFS export	A directory or filesystem made available by an NFS server for access by authorised remote clients.
NFS mount	A filesystem mount providing access to a directory exported by a remote Network File System server.
Nginx	A web server and reverse-proxy software package commonly used to serve websites, proxy applications and handle network traffic.
NIC	Network Interface Controller – hardware that provides a computer or device with network connectivity.
NIC teaming	A technique in which multiple network interfaces are combined or coordinated to provide redundancy, load distribution or increased availability.
nice	A Unix and Linux command or process property controlling the scheduling priority of a process relative to other processes.
nice value	A Unix process scheduling value influencing the relative priority with which a process competes for CPU time.
nightly build	A software build automatically produced on a regular basis, traditionally once per night, from the current development source.
Ninja	A build system designed to execute build operations quickly, particularly when generated by higher-level build tools such as CMake.
NIS	Network Information Service – a distributed system historically used on Unix networks to provide shared information such as user accounts and hostnames.
nm	A Unix utility that lists symbols contained in object files, libraries and executable programs.
NNTP	Network News Transfer Protocol – the protocol used to distribute, retrieve and post messages within the Usenet system.
NNTP Server	A server that stores and exchanges Usenet messages using the Network News Transfer Protocol.
noatime	A Linux filesystem mount option preventing normal access-time updates for file reads.
nohup	No Hang Up – a Unix and Linux command used to run a program so that it can continue operating after the controlling terminal or user session ends.
non-blocking I/O	I/O configured so that a call returns without waiting indefinitely for the requested operation to become possible.
non-interactive shell	A shell executing commands from a script or other input without an interactive user session.
non-login shell	A shell that is not started as part of a login session and therefore normally uses a different set of startup files.
non-zero exit status	A Unix/Linux exit status other than zero, conventionally indicating failure, an error or another condition requiring attention.
nonce	Number used once – a value intended to be unique within a particular cryptographic context and commonly used to prevent replay or ensure uniqueness.
nonce reuse	The reuse of a nonce where uniqueness is required by a cryptographic algorithm or protocol, potentially causing serious security failures.
NOR flash	A type of flash memory offering random-access characteristics and commonly used for firmware and boot storage.
Notification	An alert generated by an application or online service to inform a user about an event, activity, message, interaction, or other update.
NS record	Name Server record – a DNS record identifying the authoritative name servers for a DNS zone.
NSS	Name Service Switch – a mechanism used by Unix and Linux systems to configure where information such as users, groups and hostnames should be obtained.
NTFS	New Technology File System – the principal filesystem used by modern Microsoft Windows systems.
NTP	Network Time Protocol – a protocol used to synchronise computer clocks over a network.
NTP server	A network service that provides accurate time information using the Network Time Protocol, allowing computers to synchronise their system clocks.
NULL	A special database value representing the absence of a value or an unknown value. NULL is not the same as zero or an empty string.
null pointer	A pointer whose value indicates that it does not refer to a valid object or memory location.
NULL pointer dereference	An attempt to access memory through a null pointer, commonly resulting in a program crash.
NUMA	Non-Uniform Memory Access – a computer architecture in which the time required to access memory depends on which processor or memory node is involved.
NVMe	Non-Volatile Memory Express – a protocol designed specifically for high-speed access to non-volatile memory storage devices, particularly SSDs.
NVMe namespace	A logical storage area exposed by an NVMe device and identified by a namespace identifier.
NVR	Network Video Recorder – a computer-based or dedicated system that records video streams received over a network, commonly used in surveillance systems.
NX bit	No-eXecute bit – a processor-supported memory-protection feature allowing pages of memory to be marked as non-executable.
OAuth	An authorisation framework that allows applications to obtain limited access to resources without requiring the user to disclose their main password to the application.
objcopy	A GNU Binutils utility used to copy and transform object files between formats or manipulate their sections and symbols.
objdump	A GNU utility for displaying and analysing information contained in object files and executable binaries.
Object	A programming construct that combines data with operations or methods that can act upon that data.
Object Code	Machine-oriented code produced by a compiler or assembler before or as part of the process of creating an executable program.
object file	A file containing compiled or assembled machine code and associated information that has not yet necessarily been combined into a complete executable.
Object-Oriented Programming	A programming approach that organises software around objects containing data and associated behaviour.
observability	The ability to understand the internal state and behaviour of a system from information it exposes, commonly including logs, metrics and traces.
OCR	Optical Character Recognition – technology that converts text contained in images or scanned documents into machine-readable text.
OCSP	Online Certificate Status Protocol – a protocol allowing a client to query whether a digital certificate has been revoked.
one-based indexing	An indexing convention in which the first element of a sequence is assigned index one.
Online Community	A group of people who interact with one another through an Internet-based service, forum, social network, messaging system, or other online environment.
Online Service	A computer-based service accessed remotely over a communications network, particularly a term historically used for commercial services providing forums, messaging, information and other facilities.
OOB	Out Of Band – describing a management or communication path separate from the normal production data path.
OOM	Out Of Memory – a condition in which a system cannot provide sufficient memory for a requested operation or allocation.
OOM Killer	Out Of Memory Killer – a Linux kernel mechanism that terminates selected processes when the system cannot satisfy memory allocation requests.
opcode	The portion of a machine instruction identifying the operation that the processor is being asked to perform.
open file	A file or other input/output resource currently associated with an open file descriptor in a process.
open file description	The kernel data structure associated with an open file and containing information such as the current file offset and status flags.
Open Port	A network port on which a service is listening for incoming connections.
Open Source	Software whose source code is made available under a licence that permits users to inspect, modify and generally redistribute it subject to the licence terms.
Open Source Licence	A software licence that defines the rights and conditions under which source code may be used, modified and redistributed.
Open Source Software	Software distributed under a licence that makes its source code available for inspection and, depending on the licence, modification and redistribution.
OpenBSD	A free and open-source Unix-like operating system derived from BSD, particularly known for its emphasis on security, correctness and code auditing.
OpenVPN	An open-source virtual private network technology using encrypted connections to provide secure communication over untrusted networks.
operand	A value, register, memory location or other data item on which a machine instruction operates.
Operating System	Software that manages computer hardware and provides services and an interface for application programs and users.
operator	A symbol or keyword representing an operation performed on one or more values.
optimization	The process of transforming software or generated machine code to improve characteristics such as execution speed, memory usage or power consumption.
origin	In web security, the combination of scheme, hostname and port that identifies the security origin of a resource.
origin server	The server holding or generating the original version of a web resource, as distinct from a cache, proxy or CDN edge server.
orphan process	A process whose original parent process has terminated. On Unix-like systems it is adopted by another process, ultimately by PID 1 or an appropriate subreaper.
OSPF	Open Shortest Path First – a link-state interior gateway routing protocol widely used within IP networks.
out-of-band management	Management of a computer or network device using a separate management interface or network that does not depend on the normal production network path.
output redirection	Shell redirection that causes a command's standard output to be written to a file or another destination.
overlay filesystem	A filesystem mechanism that combines multiple directory trees into a single merged view, widely used by container systems.
overlayfs	The Linux implementation of an overlay filesystem, allowing a writable upper filesystem to be combined with one or more lower read-only filesystem layers.
overloading	A programming-language feature allowing the same function, method or operator name to represent different operations depending on its arguments or context.
P2P	Peer-to-Peer – a network architecture in which participating computers communicate directly with one another rather than relying entirely on a central server.
package artifact	A distributable software file produced by a build or packaging process, such as a Debian package or archive.
package cache	A local store containing downloaded software packages, commonly retained so that they can be reused without downloading them again.
package dependency	A requirement that one software package needs another package, library or component in order to function correctly.
package format	The structure and conventions used to store software files, metadata and installation information within a package.
package manager	Software used to install, update, remove and track software packages and their dependencies on an operating system.
package pinning	A package-management technique used to control which package versions or repositories are preferred during dependency resolution.
package repository	A collection of software packages and associated metadata maintained for installation or distribution by a package-management system.
Packet	A formatted unit of data carried across a computer network.
Packet Capture	The recording and examination of network packets transmitted across a network interface, commonly for troubleshooting or security analysis.
packet filter	A firewall mechanism that examines network packets and applies rules determining whether they should be allowed, rejected or discarded.
Packet Loss	The failure of network packets to reach their intended destination, potentially causing delays, reduced performance or interruptions.
PACS	Picture Archiving and Communication System – a system used by medical organisations to store, retrieve, distribute and display medical images.
page	A fixed-size unit of virtual and physical memory used by an operating system's memory-management system.
page cache	Memory used by an operating system to cache file contents, allowing subsequent accesses to be served from memory rather than storage.
page fault	An event occurring when a program accesses a virtual-memory page that is not currently available in the required physical memory location, requiring the operating system to resolve the access.
page size	The fixed amount of memory represented by one virtual-memory page.
page table	A data structure used by a memory-management system to map virtual memory pages to physical memory locations and associated permissions.
PAM	Pluggable Authentication Modules – a framework allowing Linux and Unix applications to use configurable modules for authentication and related account-management functions.
PAM module	A software component used by the Pluggable Authentication Modules framework to provide a particular authentication, account, session or password-management function.
Parallel Processing	The execution of multiple computational operations at the same time using multiple processors, cores or other resources.
parallelism	The simultaneous execution of multiple operations, normally using multiple processor cores or other parallel hardware.
parameter	A named value or variable specified by a function or procedure definition that receives a value when the function is called.
parser	Software that analyses input according to a defined grammar and converts it into a structured representation suitable for further processing.
Partition	A logically defined section of a storage device that can contain its own filesystem or be used for another storage purpose.
Partition Table	Data stored on a storage device describing how the device is divided into partitions.
passwd	Password – a Unix and Linux command used to change a user account password.
Password	A secret sequence of characters used to authenticate a user or protect access to an account or system.
password manager	Software designed to securely store and manage passwords and other authentication credentials.
PAT	Port Address Translation – a form of network address translation that distinguishes multiple connections by using transport-layer port numbers.
Patch	A software update designed to fix bugs, improve functionality or address security vulnerabilities.
patch file	A file containing changes in a standard diff format that can be applied to source code or other text files.
patch level	A version or release designation identifying a set of bug fixes or minor changes applied to a software product.
patch series	A collection of related patches intended to be applied together to modify or update source code.
patch version	The third component of a semantic version number, normally incremented for backward-compatible bug fixes.
PATH	An environment variable containing a list of directories that a shell searches when locating executable commands entered without an explicit path.
pathname expansion	Shell processing that expands wildcard patterns such as * and ? into matching filenames.
payload	The useful data carried inside a packet, frame or other protocol structure, excluding the protocol headers and control information.
PBKDF2	Password-Based Key Derivation Function 2 – a password-derived key-generation algorithm that repeatedly applies a pseudorandom function to increase the computational cost of guessing passwords.
PCI	Peripheral Component Interconnect – an older computer expansion bus standard used to connect peripheral devices to a motherboard.
PCI bus	Peripheral Component Interconnect – a computer bus standard used to connect expansion devices and controllers to a system.
PCI Express	Peripheral Component Interconnect Express – a high-speed serial expansion bus used to connect components such as graphics cards, network adapters and solid-state storage devices.
PCIe	Peripheral Component Interconnect Express – a high-speed computer expansion bus used to connect devices such as graphics cards, network adapters and NVMe storage.
PCIe lane	A single bidirectional serial communication path within a PCI Express connection.
PCIe link	A PCI Express connection consisting of one or more lanes between a root complex or switch and an endpoint device.
PCM	Pulse-Code Modulation – a method of digitally representing sampled analogue signals, commonly used for uncompressed digital audio.
PDF	Portable Document Format – a document format designed to preserve the layout and appearance of documents across different systems and applications.
Peer	A computer or participant in a peer-to-peer network that can communicate directly with other participating computers.
Peer-to-Peer	A network architecture in which participating computers can act as both clients and providers of resources or services.
Peering	The direct exchange of network traffic between independently operated networks, normally without sending that traffic through an intermediary transit provider.
performance counter	A hardware or software counter recording measurable events such as processor instructions, cache misses or branch mispredictions.
permission bit	A Unix filesystem permission flag controlling whether the owner, group or other users may perform operations such as reading, writing or executing a file.
permissive licence	An open-source software licence that generally permits broad reuse, modification and redistribution with relatively few conditions.
perror	A C-library function that prints a human-readable description of the error represented by errno.
PGID	Process Group ID – the identifier identifying a Unix process group.
PGP	Pretty Good Privacy – a family of technologies used for encrypting and digitally signing data, particularly email and files.
pgrep	A Unix and Linux command that searches for running processes matching specified criteria and displays their process IDs.
Phishing	A social engineering technique in which attackers attempt to trick people into revealing sensitive information or performing an unwanted action.
Phreaking	The exploration or manipulation of telephone systems and signalling technology, historically associated with attempts to understand or circumvent telephone-network charging and control mechanisms.
physical address	An address referring to a location in physical memory or a physical device address space.
physical volume	In LVM, a storage device or partition prepared for use as part of a volume group.
Pi Imager	Raspberry Pi Imager – software used to write operating-system images and other disk images to SD cards and USB storage for Raspberry Pi computers.
PID	Process Identifier – a numerical identifier assigned by the operating system to a running process.
PID 1	The process identifier traditionally assigned to the first userspace process started by the Linux kernel. PID 1 has special responsibilities, including adopting orphaned processes.
PID file	A file conventionally containing the process ID of a running daemon or service.
PID namespace	A Linux namespace providing processes with an isolated process-ID hierarchy.
Pine	A text-based email and Internet news client originally developed at the University of Washington and later succeeded by Alpine.
ping	A network diagnostic utility and protocol operation used to test whether a host is reachable and to measure the round-trip time of network packets.
Ping of Death	A historical type of network attack involving malformed or oversized ICMP packets intended to cause vulnerable systems to crash.
Pinned Post	A post deliberately kept in a prominent position on a profile, page, channel, or community so that it remains easily visible to visitors.
Pipe	A Unix and Linux mechanism that connects the output of one command directly to the input of another command.
pipeline	A sequence of Unix or Linux commands connected by pipes so that the output of one command becomes the input of the next.
Pipelining	A processor technique that overlaps different stages of instruction processing so that multiple instructions can be in different stages simultaneously.
pixel	The smallest addressable element of a raster image or display, normally containing colour or intensity information.
pkg-config	A utility providing compiler and linker options for software libraries by reading package metadata files.
pkgconf	A compatible implementation of the pkg-config utility used to retrieve compiler and linker flags for software libraries.
PKI	Public Key Infrastructure – the systems, policies and technologies used to manage public-key cryptography and digital certificates.
pkill	A Unix and Linux command that sends signals to processes selected according to criteria such as process name.
plaintext	Data in its original readable or usable form before encryption.
PMTUD	Path Maximum Transmission Unit Discovery – a mechanism used to determine the largest packet size that can traverse a network path without fragmentation.
PNG	Portable Network Graphics – a lossless raster image format supporting features including transparency and a wide range of colour depths.
Podcast	A series of audio or video programmes distributed over the Internet, commonly using a feed that allows software to discover and retrieve new episodes.
PoE	Power over Ethernet – a technology allowing electrical power and network data to be carried over the same Ethernet cabling.
PoE injector	A device that adds electrical power to Ethernet cabling so that a compatible network device can receive both network connectivity and power through the same cable.
pointer	A programming-language value containing a memory address or reference to another object or data structure.
pointer arithmetic	Arithmetic operations performed on pointers, normally interpreted relative to the size of the objects to which they point.
Poly1305	A message-authentication algorithm commonly combined with ChaCha20 to provide authenticated encryption.
polymorphism	The ability for different types or objects to provide compatible interfaces while implementing behaviour appropriate to their individual types.
POP	Post Office Protocol – a family of protocols designed to allow email clients to retrieve messages from a mail server.
POP3	Post Office Protocol version 3 – an email protocol commonly used to download messages from a mail server to an email client.
Port	A numbered logical endpoint used by network protocols to identify a particular service or application on a computer.
Port Forwarding	A router configuration that forwards incoming network traffic received on a particular port to a specified device and port on an internal network.
Port Number	A numerical identifier used by transport protocols to associate network traffic with a particular service or application.
port scan	A technique for probing a host to determine which network ports are open, closed or filtered.
POSIX	Portable Operating System Interface – a family of standards defining interfaces and behaviours intended to improve compatibility between Unix-like operating systems.
POSIX ACL	A Unix filesystem access-control mechanism extending traditional owner, group and other permissions with additional per-user and per-group entries.
POSIX locale	A locale environment and behaviour specification used by Unix-like systems to provide predictable character and formatting behaviour.
POSIX shell	A command shell conforming to the shell-related requirements of the Portable Operating System Interface standard, allowing scripts to be written for compatibility across Unix-like systems.
POSIX.1	The POSIX standard covering operating-system interfaces and behaviour intended to improve portability between Unix-like systems.
POST	Power-On Self-Test – a series of hardware checks performed when a computer starts to identify basic hardware problems before the operating system loads.
Post (social media)	A piece of content published by a user or account to an online service, potentially consisting of text, images, video, audio, links, or other media.
POST code	A numerical or otherwise encoded diagnostic value produced during the Power-On Self-Test to indicate the stage or result of hardware initialisation.
Post Reach	The number of distinct users or accounts to whom a social-media post has been displayed or made available.
POTS	Plain Old Telephone Service – the traditional analogue public telephone service using conventional telephone lines.
Power Supply Unit	Hardware that converts electrical power from a mains supply into the voltages required by computer components.
poweroff	An operating-system action that shuts down the system and requests that the machine power be switched off.
PowerShell	A command-line shell and scripting environment developed by Microsoft and available on Windows, Linux and macOS.
PPI	Pixels Per Inch – a measure of pixel density used for digital images and displays.
PPID	Parent Process Identifier – the process ID of the process that created or started a particular process.
PPP	Point-to-Point Protocol – a data-link protocol historically used to establish network connections over serial links, telephone lines and other point-to-point connections.
PPP over Ethernet	A method of carrying Point-to-Point Protocol frames over Ethernet, historically and currently used by some broadband Internet services.
pragma	A compiler- or language-specific directive embedded in source code to request special processing or behaviour.
preemption	The ability of an operating system to interrupt a running process or thread and allow another task to use the processor.
preemptive multitasking	A multitasking model in which the operating system can interrupt a running process or thread and give processor time to another.
prefix	A directory prefix used by Unix/Linux software installation systems to determine where programs, libraries and supporting files should be installed.
preprocessor	A program or compilation stage that transforms source code before the main compiler processes it, commonly handling macros and file inclusion.
Primary Key	A database column or combination of columns that uniquely identifies each row in a table.
Primary Storage	Computer storage directly accessible to the processor during normal operation, commonly referring to RAM and related memory.
priority	A value or classification used by an operating system or scheduler to influence the order or amount of processor time given to competing tasks.
priority inversion	A scheduling problem in which a high-priority task is indirectly prevented from running because a lower-priority task holds a resource it requires.
Private Account	A social-media account whose content is restricted so that only approved followers or authorised users can access it.
private address	An IP address from an address range reserved for use within private networks and not normally routed directly across the public Internet.
private key	A cryptographic key that must be kept secret and is used for operations such as decryption, digital signatures or authentication.
privilege escalation	A security attack or condition in which software or a user obtains privileges greater than those originally granted.
privileged instruction	A processor instruction that can only be executed in a sufficiently privileged execution mode.
procedural programming	A programming style organising software around procedures or functions that operate on data and program state.
Process	A running instance of a program together with the resources and state associated with its execution.
process group	A collection of related Unix or Linux processes that can be managed together, particularly for job control and signal handling.
process group leader	The process whose process ID is equal to the process-group ID of a Unix process group.
process ID	A numerical identifier assigned by an operating system to a running process.
process substitution	A shell feature, supported by shells such as Bash, that makes the input or output of a command available to another command as though it were a file.
process tree	A hierarchical representation showing parent and child relationships between processes.
Processor	Another common name for a CPU, the component that executes computer instructions.
procfs	Process File System – a virtual Linux filesystem exposing information about running processes and various kernel and system parameters, normally mounted at /proc.
Profile (social media)	The page or collection of information representing a user, organisation, or other account on a social-media service.
profiler	Software that measures and reports aspects of program execution, such as CPU usage, function call frequency or memory consumption.
profiling	The measurement of a program's execution to determine where processor time, memory or other resources are being consumed.
program counter	A CPU register containing the address of the next instruction to be executed, also known as the instruction pointer on many architectures.
project quota	A filesystem quota applied to a project identifier rather than directly to an individual user or group.
Protocol	A defined set of rules and conventions that allows computers and other devices to communicate with each other.
proxy	An intermediary system that receives requests from one party and makes corresponding requests to another system.
proxy cache	A cache maintained by an intermediary proxy server containing previously retrieved resources that can potentially be returned without contacting the original server.
Proxy Server	A server that acts as an intermediary between a client and another server, forwarding requests and responses on behalf of the client.
ps	Process Status – a Unix and Linux command used to display information about currently running processes.
pseudo-terminal	A software facility providing a program with an interface behaving like a physical terminal.
PSTN	Public Switched Telephone Network – the worldwide collection of interconnected public telephone networks.
PSU	Power Supply Unit – the component that supplies electrical power to the various parts of a computer.
PTR Record	A DNS record used for reverse DNS lookups to associate an IP address with a domain name.
ptrace	A Unix/Linux system call interface allowing one process to observe and control another process, commonly used by debuggers and tracing tools.
PTS	Pseudo-Terminal Slave – the slave side of a Unix pseudo-terminal pair, commonly represented under /dev/pts.
PTY	Pseudo-Terminal – a pair of virtual devices providing a terminal interface between a program and another program acting as a terminal emulator or remote connection.
public address	An IP address that is globally routable on the public Internet, subject to routing and service-provider policies.
public key	A cryptographic key intended to be distributed openly and used for operations such as encryption, signature verification or key exchange.
public key authentication	A cryptographic authentication method using a private key held by the user and a corresponding public key registered with the remote service.
pull request	A proposed set of changes submitted for review and potential integration into another branch or repository.
pure function	A function that has no observable side effects and whose result depends only on its inputs.
PUT	An HTTP method commonly used to create or replace the representation of a resource at a specified URI.
pwd	Print Working Directory – a Unix and Linux command that displays the pathname of the current working directory.
PWM	Pulse-Width Modulation – a technique in which the duty cycle of a repeating signal is varied to control power, brightness, motor speed or other analogue-like effects.
PWM fan	A cooling fan whose speed can be controlled using pulse-width modulation, commonly through a dedicated control signal.
QoS	Quality of Service – mechanisms for managing network traffic so that selected traffic receives particular treatment with respect to factors such as latency, bandwidth or packet loss.
Query	A request made to a database or other information system to retrieve or manipulate data.
QUIC	A modern transport protocol built over UDP that provides features such as encrypted connections, multiplexed streams and reduced connection-establishment latency.
quota	A configured limit on a resource such as disk storage, number of files or other consumable system resource.
Quote Post	A social-media post that republishes another post while adding the user's own commentary or other content.
race condition	A situation in which the behaviour or result of a program depends on the timing or ordering of concurrent operations.
RAID	Redundant Array of Independent Disks – a technology that combines multiple storage devices to provide improved performance, redundancy or both.
RAID 0	A RAID configuration that distributes data across multiple drives for increased performance but provides no redundancy.
RAID 1	A RAID configuration that mirrors data between drives to provide redundancy if one drive fails.
RAID 10	A RAID configuration combining disk mirroring and striping to provide redundancy and improved performance.
RAID 5	A RAID configuration that distributes data and parity information across multiple drives, allowing the array to tolerate the failure of one drive.
RAID 6	A RAID configuration using distributed dual parity that can tolerate the failure of two drives.
RAID Controller	Hardware or software responsible for managing a RAID array and presenting its storage to the operating system.
RAII	Resource Acquisition Is Initialization – a C++ programming technique in which resource lifetime is tied to object lifetime, commonly using constructors and destructors.
rainbow table	A precomputed collection of password hashes and related values used to accelerate the recovery of unsalted passwords from their hashes.
RAM	Random Access Memory – fast, volatile memory used by a computer to temporarily store programs and data currently in use.
RAM Disk	A portion of RAM configured to behave like a storage device. It can be extremely fast but its contents are normally lost when power is removed.
ramfs	A Linux RAM-based filesystem with no built-in size limit other than available memory. Unlike tmpfs, it does not support swapping its contents.
random number generator	A system or algorithm that produces values intended to have random or unpredictable characteristics.
random seed	An initial value supplied to a pseudorandom number generator from which a sequence of generated values is derived.
Ransomware	Malicious software that prevents access to data or systems, commonly by encrypting files, and demands payment from the victim.
Raspberry Pi	A family of small single-board computers developed in the UK by the Raspberry Pi Foundation. They are widely used for education, experimentation, embedded projects, home automation, media systems and lightweight servers.
Raspberry Pi OS	The official Linux-based operating system distributed for Raspberry Pi computers, based on Debian.
raster image	An image represented as a rectangular grid of individual pixels, with each pixel containing colour or intensity information.
rate limiting	A mechanism that restricts how frequently a client, user or service may perform a particular operation during a specified period.
RAW Image	A digital image file containing minimally processed data captured by an image sensor, normally retaining considerably more information for subsequent processing than a JPEG image.
raw mode	A terminal input mode in which input characters are delivered to an application with minimal processing by the terminal line discipline.
RC	Release Candidate – a software build considered a potential final release pending testing and the absence of significant remaining defects.
RDP	Remote Desktop Protocol – a protocol developed by Microsoft for providing remote graphical access to Windows computers and applications.