3 Terms and definitions [intro.defs]

3.47[defns.required.behavior]required behavior

⟨library⟩ description of replacement function ([defns.replacement]) and handler function ([defns.handler]) semantics applicable to both the behavior provided by the implementation and the behavior of any such function definition in the program
[Note 1: 
If such a function defined in a C++ program fails to meet the required behavior when it executes, the behavior is undefined.
— end note]