(in notation) the group of sharps or flats placed after the clef to indicate the tonality of the music following.
Origin
1870-75
Examples from the web for key signature
Warm water is a key signature element of a tropical ocean.
British Dictionary definitions for key signature
key signature
noun
1.
(music) a group of sharps or flats appearing at the beginning of each stave line to indicate the key in which a piece, section, etc, is to be performed