clef (klěf) n.
Music A symbol indicating the pitch represented by one line of a staff, in relation to which the other pitches of the staff can be determined.
[French, key, from Old French, from Latin clāvis.]
1576 in a musical sense, from M.Fr. clef, from L. clavis "key" (see slot (2)). The most common is the treble or G-clef, denoting the G above middle C on the piano.