Callback (computer programming)

In computer programming, a callback or callback function is any reference to executable code that is passed as an argument to another piece of code; that code is expected to call back (execute) the callback function as part of its job. This execution may be immediate as in a synchronous callback, or it might happen at a later point in time as in an asynchronous callback. They are also called blocking and non-blocking.

Programming languages support callbacks in different ways, often implementing them with subroutines, lambda expressions, blocks, or function pointers.