A callback is any executable code that is passed as an argument to other code, which is expected to call back (execute) the argument at a given time [Source : Wiki]. In simple language, If a reference of a function is passed to another function as an argument to call it, then it will be called as a Callback function.
In C, a callback function is a function that is called through a function pointer.
Below is a simple example in C to illustrate the above definition to make it more clear:
I am function A
In C++ STL, functors are also used for this purpose.
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