Nouns

(n)altoa singer whose voice lies in the alto clef
(n)contralto, altothe lowest female singing voice
(n)countertenor, altothe highest adult male singing voice
(n)alto(of a musical instrument) the second highest instrument in a family of musical instruments
(n)altothe pitch range of the lowest female voice

Adjectives

(s)alto, contraltoof or being the lowest female voice
(s)countertenor, altoof or being the highest male voice; having a range above that of tenor
(s)alto(of a musical instrument) second highest member of a group“alto clarinet or recorder”