swimsuit vs bathing costume

swimsuit and bathing costume both are nouns.

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

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

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

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