swimming costume vs swimsuit

swimming costume and swimsuit both are nouns.

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

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

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

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