n. (pl. -os) 1 = CONTRALTO. 2 a the highest adult male singing-voice, above tenor. b a singer with this voice. c a part written for it. 3 a (attrib.) denoting the member of a family of instruments pitched second- or third-highest. b an alto instrument, esp. an alto saxophone. alto clef a clef placing middle C on the middle line of the staff, used chiefly for viola music.
[ It. alto (canto) high (singing) ]