fancy dress vs disguise

fancy dress and disguise both are nouns.

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

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

fancy dress: a costume worn as a disguise at a masquerade party
disguise: any attire that modifies the appearance in order to conceal the wearer's identity

Other hypernyms of fancy dress include costume.

fancy dress (noun)	disguise (noun)
a costume worn as a disguise at a masquerade party	the act of concealing the identity of something by modifying its appearance
	any attire that modifies the appearance in order to conceal the wearer's identity
	an outward semblance that misrepresents the true nature of something

fancy dress (verb)	disguise (verb)
	make unrecognizable