costume vs garment

costume and garment both are nouns.

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

As verbs, garment is a hypernym of costume; that is, garment is a word with a broader meaning than costume:

Other hypernyms of costume include apparel, clothe, dress, enclothe, fit out, garb, habilitate, raiment, tog.

costume (noun)	garment (noun)
the attire worn in a play or at a fancy dress ball	an article of clothing
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)	garment (verb)
dress in a costume	provide with clothes or put clothes on
furnish with costumes; as for a film or play