apparel vs costume

apparel and costume both are nouns.

Word	Noun	Adjective	Verb	Adverb
apparel	Yes	No	Yes	No
costume	Yes	No	Yes	No

As verbs, costume is a hyponym of apparel; that is, costume is a word with a more specific, narrower meaning than apparel:

apparel (noun)	costume (noun)
clothing in general	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)

apparel (verb)	costume (verb)
provide with clothes or put clothes on	dress in a costume
	furnish with costumes; as for a film or play