boy vs altar boy

boy and altar boy both are nouns.

Word	Noun	Adjective	Verb	Adverb
boy	Yes	No	No	No
altar boy	Yes	No	No	No

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

boy (noun)	altar boy (noun)
a youthful male person	a boy serving as an acolyte
(ethnic slur) offensive and disparaging term for Black man
a friendly informal reference to a grown man
a male human offspring