The term costume can refer to wardrobe and dress in general, or to the distinctive style of dress of a particular people, class, or period. It can also refer to the artistic arrangement of accessories in a picture, statue, poem, or play, appropriate to the time, place, or other circumstances represented or described, or to a particular style of clothing worn to portray the wearer as a character or type of character other than their regular persona at a social event such as a fancy dress party or in an artistic theatrical performance.

Theatrical costumes, in combination with other aspects, serve to portray performers' age, gender role, profession, social class, personality, and suchlike. Sometimes theatrical costumes literally mimic what the costume designer thinks the character would wear if the character actually existed. On the other hand, often stylized theatrical costumes can exaggerate some aspect of a character.

National costume or regional costume can express local (or exiled) identity and emphasise uniqueness.

The wearing of costumes has become an important part of Halloween celebrations, and (to a lesser extent) people may also wear costumes in conjunction with other holiday celebrations, such as Christmas and Easter. Halloween costumes traditionally take the form of supernatural creatures such as ghosts, vampires, and angels, whereas Christmas and Easter costumes typically portray mythical holiday characters, such as Santa Claus or the Easter Bunny. Costumes may also serve to portray various other character themes during secular holidays, such as an Uncle Sam costume worn on the 4th of July.

Some people wear costumes for Censored page purposes. Most people consider this harmless fun, but some regard this behavior as a form of Censored page. Some say it is both.

See also: cosplay