Terminology

Short, practical definitions for newer developers. Tables list the familiar term first, also known as, then the definition.

Fundamentals

Term	Also known as	Definition
API	application programming interface	A defined interface for one program to request data or actions from another.
SDK	software development kit	A bundle of libraries, tools, and examples for working with an API or platform.
CLI	command line interface	A program you run in the terminal by typing commands to perform tasks.
LTS	long term support	A release line that keeps receiving security and bug fixes for an extended period of time.
SemVer	semantic versioning	Version numbers as MAJOR.MINOR.PATCH where breaking changes increase the MAJOR number, features increase the MINOR number, and fixes increase the PATCH number.

Packages and Dependencies

Term	Also known as	Definition
Dependency	runtime dep	A package your app needs when it runs.
Development dependency	dev dep	A package used for build, test, or tooling, not at runtime.
Peer dependency	peer dep	A package your project must install because a library you use expects to use your copy, with a compatible version.
lockfile	package lock	A file that pins exact versions so installs are repeatable.

JavaScript and TypeScript

Term	Also known as	Definition
ESM	ECMAScript Modules	Modern JavaScript modules that use import and export.
CommonJS	CJS	Older Node.js modules that use require and module.exports.
compile	—	Translate source code to a lower-level form such as machine code or bytecode (e.g., C++ → machine code, Python → bytecode).
transpile	source-to-source	Convert code to a similar-level language or version (e.g., TypeScript → JavaScript).
polyfill	shim	Runtime code that adds a missing browser feature or platform API.
type narrowing	refinement	Make a type more specific after a check (e.g., typeof, in, truthy checks).
union type	A | B	A value can be one of several types (e.g., soup OR salad).
intersection type	A & B	A value must satisfy multiple types at once (e.g., numbers that are even AND prime).
nominal typing	name based typing	Requires the same declared type or an explicit relation (e.g., UserId is not assignable to PostId even if both are string).
structural typing	duck typing	Compatible by shape, not by name (e.g., { x: number; y: number } is assignable to Coordinates).
ambient types	global types	Type declarations available globally without imports.
any vs unknown	—	any disables type safety; unknown forces explicit type checking before use.
never	impossible type	A type with no possible values.

Builds and Tooling

Term	Also known as	Definition
bundle	build artifact	Combine many source files into one or a few files the browser or Node.js can load.
minify	—	Remove comments and whitespace and shorten names to shrink files.
tree shaking	dead code elimination	Remove code that is never imported or used.
code splitting	lazy chunks	Break the app into chunks and load them on demand or by route.
source map	.map file	Link compiled or minified code back to the original files for debugging.
watch mode	auto rebuild	Rebuild automatically when files change.
hot reload	HMR	Swap updated modules into a running app without a full page reload, often preserving state.

Module Behavior

Term	Also known as	Definition
exports field	package exports	A map of entry points that a package exposes to importers (projects that use that package).
conditional exports	env specific exports	Choose different entry files by environment or import kind (e.g., import → index.mjs, require → index.cjs).
side-effect import	setup import	Run the module's top-level code and import nothing (e.g., import './polyfills';).
value named import	named import	Import specific exports as bindings (e.g., import { parse } from './util';).
value default import	default import	Import the module's default export as one binding (e.g., import parse from './util';).
type-only import	type import	Import types only and erase them at build time in TypeScript (e.g., import type { User } from './types';).
top level await	TLA	Use await in an ESM module's top scope.

Runtime and Browser

Term	Also known as	Definition
event loop	—	The system that runs one task at a time and schedules what runs next.
blocking code	synchronous stall	Long synchronous work that stops the event loop so clicks and timers wait.
hydration	reattach on client	Client JavaScript takes over server-rendered HTML and wires up events.
CSR	client side rendering	Render the UI in the browser after the JavaScript bundle loads.
SSR	server side rendering	Render HTML on the server for each request, then hydrate in the browser.
SSG	static site generation	Pre-render HTML at build time and serve static files.
CORS	cross origin requests	Browser rule that blocks cross-site requests unless the server opts in with CORS headers.
CSP	content security policy	Browser rule that allows only approved scripts and resources.

Git and Workflow

Term	Also known as	Definition
commit	revision	A saved snapshot of changes in the repository.
branch	—	A movable line of work that points to a commit.
merge	merge commit	Combine changes from one branch into another, creating a merge commit if needed.
rebase	—	Move commits to a new base and replay them to keep history linear.
fast forward	FF merge	A merge that only moves the branch pointer to a later commit with no new commit.
tag	release tag	A named pointer to a specific commit, often used for releases.

Testing

Term	Also known as	Definition
unit test	spec, component test	Tests one function or module in isolation with dependencies mocked.
integration test	—	Tests how multiple parts work together across real boundaries.
end to end test	E2E	Drives the app like a user from UI to backend in a real environment.
mock	stub, fake	A stand-in function or object with controlled behavior for a dependency.
fixture	test data	Fixed sample data or files used to set up a repeatable test.
flaky test	nondeterministic test	Sometimes passes and sometimes fails without code changes, often due to timing or external systems.

Performance and UX

Term	Also known as	Definition
debounce	—	Wait to run until input has been idle for a short time (e.g., search box).
throttle	rate limit	Run at most once every set interval (e.g., scroll handler).
critical path	—	Assets that must load before the page can render or accept input.
render blocking	blocking resource	A resource that delays first paint until it finishes (often CSS or synchronous JavaScript).
a11y	accessibility	Practices that make the app usable with keyboard and assistive tech.
i18n	internationalization	Prepare for multiple locales by extracting strings and using locale-aware formats.
l10n	localization	Provide locale-specific content and formatting for a target audience.
t9n	translation	Translate UI strings between languages.

Security

Term	Also known as	Definition
XSS	cross site scripting	Attacker supplied script runs in the user's browser due to unsafe or unescaped content.
CSRF	cross site request forgery	A logged in user's browser is tricked into sending a request to your site using their credentials.
sanitization	input cleansing	Remove or escape untrusted input before using it in HTML, URLs, queries, or file paths.