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]