bikini vs bathing costume

bikini and bathing costume both are nouns.

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

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

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

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