JavaScript
In plain terms
The programming language of the web browser, now also used on servers.
Going deeper
JavaScript runs in every browser to make pages interactive, and via Node.js it also powers backends. It's dynamically typed, event-driven, and famously flexible (sometimes to a fault), making it the most ubiquitous language in software.
Under the hood
Single-threaded with an event loop; async work uses callbacks, promises, and async/await. Modern engines (V8) JIT-compile it to near-native speeds.
Examples
React/Vue frontends, Node.js APIs, Electron desktop apps, browser DOM scripting.
Think of it like...
Like the electricity in a house: it's wired into every room (browser) by default, so anything that needs to 'come alive' on a page runs on it.