Callback
In plain terms
A function passed to another function to be called later.
Going deeper
Callbacks let code run after an operation finishes, the basis of asynchronous programming before promises. Deeply nested callbacks can become hard to read ('callback hell').
Under the hood
Inversion of control where you hand off a function to be invoked later; deeply nested callbacks ('callback hell') motivated promises and async/await.
Examples
setTimeout(fn), event handlers, array.map(fn), Node.js fs.readFile callbacks.
Think of it like...
Like leaving your number with a shop: 'call me back when my order is ready,' so you don't stand waiting at the counter.