fancy dress vs costume

fancy dress and costume both are nouns.

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

As nouns, costume is a hypernym of fancy dress; that is, costume is a word with a broader meaning than fancy dress:

Other hypernyms of fancy dress include disguise.

fancy dress (noun)	costume (noun)
a costume worn as a disguise at a masquerade party	the attire worn in a play or at a fancy dress ball
	the attire characteristic of a country or a time or a social class
	unusual or period attire not characteristic of or appropriate to the time and place
	the prevalent fashion of dress (including accessories and hair style as well as garments)

fancy dress (verb)	costume (verb)
	dress in a costume
	furnish with costumes; as for a film or play