from
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]