What does alto mean?
The definition of alto
Al”to, n.; pl. Altos.
Etym: [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.