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.

Source : WordNet®

alto
     adj 1: of or being the lowest female voice [syn: {contralto}]
     2: of or being the highest male voice; having a range above
        that of tenor [syn: {countertenor}]
     3: (of a musical instrument) second highest member of a group;
        "alto clarinet or recorder"

alto
     n 1: a singer whose voice lies in the alto clef
     2: the lowest female singing voice [syn: {contralto}]
     3: the highest adult male singing voice [syn: {countertenor}]
     4: the pitch range of the lowest female voice