evening dress vs eveningwear

evening dress and eveningwear both are nouns.

Word	Noun	Adjective	Verb	Adverb
evening dress	Yes	No	No	No
eveningwear	Yes	No	No	No

As nouns, evening dress and eveningwear are synonyms defined as:

evening dress and eveningwear: attire to wear on formal occasions in the evening

Other synonyms of evening dress include evening clothes, formalwear.

evening dress (noun)	eveningwear (noun)
attire to wear on formal occasions in the evening	attire to wear on formal occasions in the evening