bikini vs swimsuit

bikini and swimsuit both are nouns.

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

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

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

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