castrato vs singer

castrato and singer both are nouns.

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

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

castrato: a male singer who was castrated before puberty and retains a soprano or alto voice
singer: a person who sings

Other hypernyms of castrato include vocaliser, vocalist, vocalizer.

castrato (noun)	singer (noun)
a male singer who was castrated before puberty and retains a soprano or alto voice	a person who sings