bathing suit vs swimming costume

bathing suit and swimming costume both are nouns.

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

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

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

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

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