singer vs alto

singer and alto both are nouns.

Word	Noun	Adjective	Verb	Adverb
singer	Yes	No	No	No
alto	Yes	Yes	No	No

As nouns, alto is a hyponym of singer; that is, alto is a word with a more specific, narrower meaning than singer:

singer (noun)	alto (noun)
a person who sings	the pitch range of the lowest female voice
	(of a musical instrument) the second highest instrument in a family of musical instruments
	the lowest female singing voice
	the highest adult male singing voice
	a singer whose voice lies in the alto clef

singer (adjective)	alto (adjective)
	(of a musical instrument) second highest member of a group
	of or being the highest male voice; having a range above that of tenor
	of or being the lowest female voice