B2[C or U]the set of clothes typical of a particular country or period of history, or suitable for a particular activity: Singers performing Mozart's operas often dress in/wear historical costume.The dancers leading the procession were in colourful and elaborate costumes.UKThe shop has a good selection of bikinis and bathing/swimming costumes.Clothes worn for a particular job or activityB2[C](UK alsofancy-dress costume)a set of clothes worn in order to look like someone or something else, especially for a party or as part of an entertainment: Our host was wearing a clown costume.The children were dressed in Halloween costumes.Clothes worn for a particular job or activity