swimwear vs bikini

swimwear and bikini both are nouns.

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

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

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

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