swimsuit vs bathing suit

swimsuit and bathing suit both are nouns.

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

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

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

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