disguise vs fancy dress

disguise and fancy dress both are nouns.

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

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

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

Other hyponyms of disguise include masquerade, masquerade costume, mask.

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

disguise (verb)	fancy dress (verb)
make unrecognizable