3 Terms and definitions [intro.defs]

3.48[defns.replacement]replacement function

⟨library⟩ non-reserved function whose definition is provided by a C++ program
[Note 1:
Only one definition for such a function is in effect for the duration of the program's execution, as the result of creating the program ([lex.phases]) and resolving the definitions of all translation units ([basic.link]).
— end note]