swimwear vs bathing costume

swimwear and bathing costume both are nouns.

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

As nouns, swimwear and bathing costume are synonyms defined as:

Other synonyms of swimwear include bathing suit, swimming costume, swimsuit.

swimwear (noun)	bathing costume (noun)
tight fitting garment worn for swimming	tight fitting garment worn for swimming