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:

In C programming, a callback is a mechanism that allows a function to call another function as an argument. A callback function is simply a function that is passed as a parameter to another function. When the main function calls the second function, it will pass the callback function as an argument. Once the second function is executed, it will call the callback function.

The use of callbacks is very common in C programming. For example, if you are writing a program that needs to sort an array of data, you might want to pass a comparison function as a callback to the sorting function. The sorting function will then use the callback function to determine the order of the data.

Another example of the use of callbacks in C programming is in event handling. When an event occurs, such as a mouse click or a key press, the program will call a function that handles the event. This function will then call the appropriate callback function to handle the event.

Callbacks provide a powerful way to extend the functionality of a program without modifying the original code. By passing a callback function as a parameter, you can add new functionality to an existing program without having to rewrite the entire program. This makes callbacks a valuable tool for software development