clef

n. sign placed upon a staff to indicate the pitch of notes (Music)

For other senses of this word, see clef (disambiguation). A clef (from the French for "key") is a musical symbol used to indicate the pitch of written notes.* Placed on one of the lines at the beginning of the staff, it indicates the name and pitch of the notes on that line. This line serves as a reference point by which the names of the notes on any other line or space of the staff may be determined.
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Noun
1. a musical notation written on a staff indicating the pitch of the notes following it
(hypernym) musical notation
(hyponym) treble clef, G clef

n. clef, key; pivotal

(n.)
A character used in musical notation to determine the position and pitch of the scale as represented on the staff.