contralto vs singer

contralto and singer both are nouns.

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

As nouns, singer is a hypernym of contralto; that is, singer is a word with a broader meaning than contralto:

Other hypernyms of contralto include vocaliser, vocalist, vocalizer.

contralto (noun)	singer (noun)
the lowest female singing voice	a person who sings
a woman singer having a contralto voice

contralto (adjective)	singer (adjective)
of or being the lowest female voice