In show business (music, TV shows, etc.), a guest appearance is a participation of an outsider performer (musician, actor, etc.), usually called guest artist (guest dancer, guest violinist, etc.), in an event (music record, concert, show, etc.), i.e., the participation of a performer which does not belong to the regular crew (cast, band, contract personnel, etc.)
In performance art, the terms guest role or guest star are also common, the latter term specifically indicating the guest appearance of a celebrity. The latter is often also credited as “Special guest star” or “Special Musical Guest Star” by some production companies.