bikini vs swimming costume

bikini and swimming costume both are nouns.

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

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

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

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