The Collaborative International Dictionary of English v.0.48:

Clef \Clef\ (kl[e^]f; 277), n. [F. clef key, a key in music, fr.
   L. clavis key. See Clavicle.] (Mus.)
   A character used in musical notation to determine the
   position and pitch of the scale as represented on the staff.
   [1913 Webster]

   Note: The clefs are three in number, called the C, F, and G
         clefs, and are probably corruptions or modifications of
         these letters. They indicate that the letters of
         absolute pitch belonging to the lines upon which they
         are placed, are respectively C, F, and G. The F or bass
         clef, and the G or treble clef, are fixed in their
         positions upon the staff. The C clef may have three
         positions. It may be placed upon the first or lower
         line of the staff, in which case it is called soprano
         clef, upon the third line, in which case it called alto
         clef, or upon the fourth line, in which case tenor
         clef. It rarely or never is placed upon the second
         line, except in ancient music. See other forms of C
         clef under C, 2.
         [1913 Webster]

   Alto clef, Bass clef. See under Alto, Bass.
      [1913 Webster]

WordNet (r) 3.0 (2006):

clef
    n 1: a musical notation written on a staff indicating the pitch
         of the notes following it