swimsuit vs swimming costume

swimsuit and swimming costume both are nouns.

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

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

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

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