Costume is a set of clothes that someone is wearing in order to look in a way that is not the same as another or as before. Some costumes are special to countries or regions. They are worn because people are proud of their country. Actors in a theatre wear costumes because they are pretending to be a person in a play or ballet. Costumes can be worn for carnivals or parties.
National or regional costume is worn to show that people come from a country or region. Many years ago, national costume was worn every day by many groups of people. Today most people do not wear costumes. Usually people dress up in national costume for special celebrations or for national dancing.
When actors perform in the theatre they usually wear a costume. These are the special clothes they wear in order to pretend they are a particular character in the story which they act out. They might be pretending to be any kind of person: a beggar, king, businessman, ancient god, etc. Some theatrical costumes stretch the truth about characters. Examples are Harlequin and Pantaloon in the Commedia dell'arte.