Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. An alto singer.

   {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.