3 Terms and definitions [intro.defs]

3.50[defns.required.behavior]required behavior

⟨library⟩ description of replacement function and handler function 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]