dress in a costume We dressed up for Halloween as pumpkins furnish with costumes; as for a film or play the attire worn in a play or at a fancy dress ball he won the prize for best costume the attire characteristic of a country or a time or a social class he wore his national costume unusual or period attire not characteristic of or appropriate to the time and place in spite of the heat he insisted on his woolen costume the prevalent fashion of dress (including accessories and hair style as well as garments)