costume vs fancy dress

costume and fancy dress both are nouns.

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

As nouns, fancy dress is a hyponym of costume; that is, fancy dress is a word with a more specific, narrower meaning than costume:

Other hyponyms of costume include masquerade, masquerade costume, fursuit.

costume (noun)	fancy dress (noun)
the attire worn in a play or at a fancy dress ball	a costume worn as a disguise at a masquerade party
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)

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