In music, the word “alto” means the second highest voice in a song. It may also refer to the second string in the guitar that is tuned in slightly higher than the third voice or slightly lower than the first voice. In normal cases, soprano and alto are sung by females but in some cases, males can still do it.
In instruments, “Alto” may also refer to corresponding vocal range whereby a certain instrument can play the alto vocal. For example in flutes and trombones, this second voice can be well played and clearly pronounced. Other players can easily assume other roles in music.
In piano and guitars, alto is integrated within the chords and it’s not easy to single it out.