bathing costume vs bathing suit

bathing costume and bathing suit both are nouns.

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

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

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

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

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