A function passed as an argument to another function, to be executed later. Callbacks are the foundation of asynchronous JavaScript and are used extensively in event handling and array methods.

Syntactic sugar built on top of Promises that makes asynchronous code look and behave like synchronous code. async functions always return a Promise, and await pauses execution until a Promise resolves.

The mechanism that allows JavaScript to perform non-blocking operations despite being single-threaded. It continuously checks if the call stack is empty, then moves tasks from the callback queue to the call stack for execution.