From The Art and Popular Culture Encyclopedia
A cameo role or cameo appearance (often shortened to just cameo) is a brief appearance of a known person in a work of the performing arts, such as plays, films, video games and television. Such a role needs not be filled by an actor: short appearances by film directors, politicians, athletes, musicians, and other celebrities are common. These roles are generally small, and most of them non-speaking.
Originally, the phrase cameo referred the famous person playing no character but him or herself. Like a cameo brooch- an engraving of a person's head or neck in relief - the actor or celebrity is instantly recognizable as him or herself, and does not play a role. More recently, "cameo" has come to refer to short appearances, whether as a character or as oneself.
Cameos are often uncredited due to their brevity or because of a perceived mismatch between the celebrity’s stature and the film or TV show he or she is appearing in. Many are publicity stunts. Others are acknowledgements of an actor’s contribution to an earlier work, as in the case of many film adaptations of TV series, or of remakes of earlier films. Others honor artists or celebrities known for work in a particular field.