bathing suit vs bathing costume

bathing suit and bathing costume both are nouns.

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

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

bathing suit and bathing costume: tight fitting garment worn for swimming

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

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