swimsuit vs bikini

swimsuit and bikini both are nouns.

Word	Noun	Adjective	Verb	Adverb
swimsuit	Yes	No	No	No
bikini	Yes	No	No	No

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

Other hyponyms of swimsuit include two-piece, maillot, tank suit, bathing trunks, swimming trunks.

swimsuit (noun)	bikini (noun)
tight fitting garment worn for swimming	a woman's very brief bathing suit