bikini vs swimwear

bikini and swimwear both are nouns.

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

As nouns, swimwear is a hypernym of bikini; that is, swimwear is a word with a broader meaning than bikini:

Other hypernyms of bikini include bathing costume, bathing suit, swimming costume, swimsuit.

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