aunt vs maiden aunt

aunt and maiden aunt both are nouns.

Word	Noun	Adjective	Verb	Adverb
aunt	Yes	No	No	No
maiden aunt	Yes	No	No	No

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

Other hyponyms of aunt include grandaunt, great-aunt.

aunt (noun)	maiden aunt (noun)
the sister of your father or mother; the wife of your uncle	an unmarried aunt