A function call is analogous to calling someone on a telephone, asking her a question, getting an answer, and hanging up; adding a callback changes the analogy so that after asking her a question, you also give her your name and number so she can call you back with the answer. -- Paul Jakubik, "Callback Implementations in C++"
You can only use call back in the sentence "What/when is the best time to call back?" You should use callback or call-back in Request a callback or The salesman did a callback at 15:03 but it was unanswered where 'callback' is a noun describing the event of calling someone back.
Call me back when the vet is ready to speak with me, and in the meantime I'll continue working on the other things on my todo list." In real life situations you should probably be a little bit more graceful, but when writing software, you can be as rude as you want to the CPU.
How to explain callbacks in plain english? How are they different from ...
A callback in C is a function that is provided to another function to "call back to" at some point when the other function is doing its task. There are two ways that a callback is used: synchronous callback and asynchronous callback.
Note: Most of the answers cover function pointers which is one possibility to achieve "callback" logic in C++, but as of today not the most favourable one I think. What are callbacks (?) and why to use them (!) A callback is a callable (see further down) accepted by a class or function, used to customize the current logic depending on that callback. One reason to use callbacks is to write ...