What is a callback function?

Question

What is a callback function?

Answer 1

A callback function is one that should be called when a certain condition is met. Instead of being called immediately, the callback function is called at a certain point in the future.

Typically it is used when a task is being started that will finish asynchronously (ie will finish some time after the calling function has returned).

For example, a function to request a webpage might require its caller to provide a callback function that will be called when the webpage has finished downloading.