Glossary

A comprehensive glossary of terms used throughout this knowledge base.

A

Term	Definition
ACID	Atomicity, Consistency, Isolation, Durability — properties guaranteeing reliable database transactions
ALU	Arithmetic Logic Unit — digital circuit performing arithmetic and bitwise operations
API	Application Programming Interface — set of protocols for building software
ARP	Address Resolution Protocol — maps IP addresses to MAC addresses
ASLR	Address Space Layout Randomization — security technique randomizing memory addresses
AVX	Advanced Vector Extensions — SIMD instruction set for x86 CPUs

B

Term	Definition
B+ Tree	Self-balancing tree structure optimized for disk-based indexing
BBR	Bottleneck Bandwidth and Round-trip propagation time — TCP congestion control algorithm
BCNF	Boyce-Codd Normal Form — stricter version of 3NF
BIOS	Basic Input/Output System — firmware for hardware initialization
BGP	Border Gateway Protocol — protocol for routing between autonomous systems
Btrfs	B-tree file system — modern Linux copy-on-write filesystem

C

Term	Definition
Cache	Small, fast memory storing frequently accessed data
CAP Theorem	States a distributed system can guarantee only 2 of: Consistency, Availability, Partition tolerance
CAS	Compare-and-Swap — atomic operation for lock-free programming
CDN	Content Delivery Network — distributed servers for content delivery
CIDR	Classless Inter-Domain Routing — IP address allocation method
CISC	Complex Instruction Set Computing
COW	Copy-on-Write — optimization deferring copies until modification
CFS	Completely Fair Scheduler — Linux default process scheduler (being superseded by EEVDF)
Cgroup	Control Group — kernel facility limiting/accounting CPU, memory and I/O for process groups
CPU	Central Processing Unit
Core Dump	On-disk image of a process’s memory written on abnormal termination (e.g. SIGSEGV) for post-mortem debugging
CTE	Common Table Expression — named temporary result set in SQL

D

Term	Definition
Daemon	Background process detached from any controlling terminal, usually started at boot
DMA	Direct Memory Access — allows hardware to access memory without CPU
DNS	Domain Name System — translates domain names to IP addresses
DRAM	Dynamic Random-Access Memory
DAC	Discretionary Access Control

E

Term	Definition
EEPROM	Electrically Erasable Programmable Read-Only Memory
EEVDF	Earliest Eligible Virtual Deadline First — Linux CFS replacement scheduler (6.6+)
epoll	Linux I/O event notification facility
Errno	Thread-local error code set by system calls on failure (see `errno(3)`)
Exec	Replacing the current process image with a new program (`execve(2)`) — the process PID survives
Exit Status	8-bit value a process reports to its parent on termination (`$?` in shells); 0 = success by convention
ext4	Fourth extended filesystem — default Linux filesystem

F

Term	Definition
FCFS	First-Come, First-Served scheduling
FIFO	First-In, First-Out
File Descriptor	Non-negative integer indexing a process’s open-file table (files, sockets, pipes)
Fork	Duplicating the calling process — child gets a copy of the address space and inherits file descriptors
FUSE	Filesystem in Userspace
FLP	Fischer-Lynch-Paterson impossibility result

G

Term	Definition
GiST	Generalized Search Tree — PostgreSQL index type
GIN	Generalized Inverted Index — PostgreSQL index type
GPU	Graphics Processing Unit

H

Term	Definition
HBM	High Bandwidth Memory
HDD	Hard Disk Drive
Huge Pages	Large memory pages (2MB or 1GB) reducing TLB misses
HTTP	Hypertext Transfer Protocol

I

Term	Definition
Init (PID 1)	First userspace process; ancestor of all processes, reaps orphans and never stops by default
I/O	Input/Output
ICMP	Internet Control Message Protocol
IEEE 754	Standard for floating-point arithmetic
IOPS	Input/Output Operations Per Second
ISA	Instruction Set Architecture
ISR	Interrupt Service Routine

J

Term	Definition
Job Control	Shell facility for managing process groups: foreground/background, `Ctrl-Z`, `fg`, `bg`, `jobs`

K

Term	Definition
Kernel	Core component of an OS managing hardware and system resources

L

Term	Definition
LRU	Least Recently Used — cache/page replacement policy
LFU	Least Frequently Used
LSM	Log-Structured Merge-tree — storage engine data structure
LKM	Loadable Kernel Module

M

Term	Definition
MAC	Media Access Control address
MESI	Modified, Exclusive, Shared, Invalid — cache coherence protocol
Mmap	Memory-mapped file
MLFQ	Multilevel Feedback Queue scheduling
MVCC	Multi-Version Concurrency Control

N

Term	Definition
NAT	Network Address Translation
Namespace	Kernel isolation feature (pid, net, mnt, uts, ipc, user, cgroup) making processes see a private system view
NUMA	Non-Uniform Memory Access
NVMe	Non-Volatile Memory Express
NVM	Non-Volatile Memory

O

Term	Definition
OOM Killer	Out-Of-Memory Killer — kernel mechanism selecting and SIGKILLing processes under memory exhaustion
OOO	Out-of-Order execution
Orphan Process	Process whose parent died; re-parented to init (PID 1) or a subreaper, which reaps it
OSPF	Open Shortest Path First — routing protocol

P

Term	Definition
PCB	Process Control Block
PCIe	Peripheral Component Interconnect Express
Paxos	Consensus algorithm for distributed systems
PID	Process Identifier — the kernel-assigned unique ID of a process or process-group leader
POSIX	Portable Operating System Interface
Process Group	Set of related processes sharing a PGID for job-control signal delivery
PTY	Pseudo Terminal — kernel device pair emulating a terminal (ssh, tmux, containers)

Q

Term	Definition
QUIC	Quick UDP Internet Connections — transport protocol

R

Term	Definition
RAID	Redundant Array of Independent Disks
RISC	Reduced Instruction Set Computing
Raft	Understandable consensus algorithm
Reaping	Parent reading a terminated child’s exit status via `wait()`/`waitpid()`, freeing the zombie’s kernel slot
RIP	Routing Information Protocol

S

Term	Definition
Session	Process-group collection tied to a controlling terminal, led by a session leader (see `setsid(2)`)
SIGCHLD	Signal sent to a parent when a child stops, resumes or terminates
SIGHUP	Hangup signal — controlling terminal closed; daemons use it to reload configuration
SIGINT	Interrupt signal from `Ctrl-C`; catchable and default-terminating
SIGKILL	Uncatchable, unignorable termination signal (9) — the kernel’s last-resort kill
SIGSEGV	Signal raised on invalid memory access; default action terminates with a core dump
SIGTERM	Default termination request (15) — polite, catchable; the standard `kill` signal
Signal	Asynchronous notification delivered to a process (numbered, with a disposition: default/ignore/handler)
Signal Disposition	Per-signal process-wide policy: default action, ignore (`SIG_IGN`), or installed handler
Signal Mask	Set of signals temporarily blocked from delivery; inspected/changed via `sigprocmask(2)`/`pthread_sigmask(3)`
SLAB	Linux kernel memory allocator
SMP	Symmetric Multiprocessing
SMT	Simultaneous Multithreading (Hyperthreading)
SRAM	Static Random-Access Memory
SSD	Solid State Drive
STAR	Situation, Task, Action, Result — behavioral interview framework

T

Term	Definition
TCP	Transmission Control Protocol
TLB	Translation Lookaside Buffer
TLS	Transport Layer Security
TSC	Time Stamp Counter
TTY	Teletypewriter — terminal device abstraction; a process’s controlling terminal drives its signals

U

Term	Definition
UDP	User Datagram Protocol
UEFI	Unified Extensible Firmware Interface

V

Term	Definition
VFS	Virtual File System
VM	Virtual Machine
VPN	Virtual Private Network
VPC	Virtual Private Cloud

W

Term	Definition
Wait / waitpid	System calls blocking a parent until a child changes state and returning its exit status
WAL	Write-Ahead Logging
WSS	Working Set Size

X

Term	Definition
XFS	High-performance filesystem

Z

Term	Definition
ZFS	Zettabyte File System
ZAB	ZooKeeper Atomic Broadcast
Zombie Process	Terminated process whose exit status the parent has not reaped yet; only its kernel PCB remains