A cameo is a brief appearance by a well-known person in a film, series, music video or show, often limited to a single scene and designed to be spotted. The word first meant a gem or shell carved in relief, a tiny portrait in miniature; Merriam-Webster traces that sense to the 15th century, with the stage and screen meaning arriving much later.
The undisputed master was Alfred Hitchcock, who turned up in 39 of his films, starting with The Lodger in 1927. He said it began for practical reasons, simply to fill the screen, before it became a superstition and then a gag. Because audiences started hunting for him, he tried to appear within the first few minutes, so viewers could stop searching and follow the plot. Among the favourites: leaving a pet shop with two small dogs in The Birds and carrying a double bass in Strangers on a Train.