'Alto clef' definitions:

Definition of 'alto clef'

noun

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

From: GCIDE

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]

From: GCIDE

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]