bikini vs bathing suit

bikini and bathing suit both are nouns.

Word	Noun	Adjective	Verb	Adverb
bikini	Yes	No	No	No
bathing suit	Yes	No	No	No

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

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

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