A callback function is a commonly used term in programming that refers to a function which is called by another function when a particular event occurs. There are several synonyms that can be used for this term, such as event handler, callback routine, callback procedure, callback method, and callback subroutine. All these terms essentially refer to the same concept, that is, a function that is registered to be called later when a specific event takes place. Whether it is known as a callback function or any of these synonyms, the purpose of this type of function is to provide a flexible way to execute code when certain actions occur within a program.