The Collaborative International Dictionary of English v.0.48:

Counter tenor \Coun"ter ten`or\ (t?n`?r). [OF. contreteneur. Cf.
   Contratenor, and see Tenor a part in music.] (Mus.)
   One of the middle parts in music, between the tenor and the
   treble; high tenor.
   [1913 Webster]

   Counter-tenor clef (Mus.), the C clef when placed on the
      third line; -- also called alto clef.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Alto \Al"to\, n.; pl. Altos. [It. alto high, fr. L. altus. Cf.
   Alt.]
   1. (Mus.) Formerly the part sung by the highest male, or
      counter-tenor, voices; now the part sung by the lowest
      female, or contralto, voices, between in tenor and
      soprano. In instrumental music it now signifies the tenor.
      [1913 Webster]

   2. An alto singer.
      [1913 Webster]

   Alto clef (Mus.) the counter-tenor clef, or the C clef,
      placed so that the two strokes include the middle line of
      the staff. --Moore.
      [1913 Webster]

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):

alto clef
    n 1: a clef that puts middle C on the third line of a staff
         [syn: alto clef, viola clef]