I. al‧to¹ /ˈæltəʊ $ -toʊ/ adjective
an alto instrument or voice produces notes at the second highest level, below a soprano:
an alto sax

[TahlilGaran] Dictionary of Contemporary English ▲

II. alto² noun (plural altos)
[Date: 1500-1600; Language: Italian; Origin: 'high', from Latin altus; ⇒ altitude]

1. [countable] a singing voice that is lower than a soprano, or a singer with a voice like this

2. [singular] the part of a musical work that is written for an alto voice or instrument
⇒ baritone¹(2), bass¹(2), soprano¹(2), tenor¹(2)

[TahlilGaran] Dictionary of Contemporary English ▲