bathing costume vs swimsuit

bathing costume and swimsuit both are nouns.

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

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

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

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