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]