- 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. - An alto singer.
Meaning of 'alto' (Princeton's WordNet)
1 . alto
[ s]
Meaning (1): - (of a musical instrument) second highest member of a group