fancy dress vs masquerade costume

fancy dress and masquerade costume both are nouns.

Word	Noun	Adjective	Verb	Adverb
fancy dress	Yes	No	No	No
masquerade costume	Yes	No	No	No

As nouns, fancy dress and masquerade costume are synonyms defined as:

fancy dress and masquerade costume: a costume worn as a disguise at a masquerade party

Other synonyms of fancy dress include masquerade.

fancy dress (noun)	masquerade costume (noun)
a costume worn as a disguise at a masquerade party	a costume worn as a disguise at a masquerade party