runtime

A runtime is the engine that actually runs your code. Your source files are just text until something reads them and brings them to life — Node.js runs JavaScript on a server, the browser runs it on a page, and the JVM runs Java. The runtime is the stage your program performs on.

Knowing which runtime you're targeting matters: the same JavaScript can have different abilities in Node.js than in a browser, because each provides its own surroundings and built-in features.

'Runtime' also means a moment in time. Errors split into ones caught early — at 'compile time', before the program runs — and ones that only surface 'at runtime', while it's actually running and a bad value finally trips it up.