al·to / ˈaltō/ • n. (pl. -tos) Mus. a voice, instrument, or part below the highest range and above tenor, in particular: ∎  the highest adult male singing voice; countertenor. ∎  the lowest female singing voice; contralto. ∎  [as adj.] denoting the member of a family of instruments pitched second or third highest: alto flute. ∎  an alto instrument, esp. an alto saxophone.

alto In singing, the highest male voice, also called countertenor; or the lowest female voice, also called contralto. It is also used to describe that member of a family of instruments with a range that corresponds to the alto voice; for example, an alto flute is a fourth lower than a standard one.

alto (It.). High.
1. Usually high type of falsetto male v., much used in Eng. church mus.; thus in SATB, A stands for alto.

2. Low-register female v., usually referred to as contralto.

3. Applied to instr., the 2nd or 3rd highest of the family.

4. (Fr.). Viola.

alto highest male voice, counter-tenor. XVII. — It. alto high — L. altus.