3 Terms and definitions [intro.defs]

3.25[defns.handler]handler function

⟨library⟩ non-reserved function whose definition may be provided by a C++ program
[Note 1: 
A C++ program may designate a handler function at various points in its execution by supplying a pointer to the function when calling any of the library functions that install handler functions (see [support]).
— end note]