dinner dress vs evening gown

dinner dress and evening gown both are nouns.

Word	Noun	Adjective	Verb	Adverb
dinner dress	Yes	No	No	No
evening gown	Yes	No	No	No

As nouns, dinner dress and evening gown are synonyms defined as:

Other synonyms of dinner dress include dinner gown, formal.

dinner dress (noun)	evening gown (noun)
a gown for evening wear	a gown for evening wear