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]