Yes, alto is a Scrabble word!

Adjective

1. Of or being the lowest female voice

2. Of or being the highest male voice; having a range above that of tenor

3. (of a musical instrument) second highest member of a group

Noun

1. A singer whose voice lies in the alto clef

2. The lowest female singing voice

3. The highest adult male singing voice

4. (of a musical instrument) the second highest instrument in a family of musical instruments

5. The pitch range of the lowest female voice