When is an actor's appearence considered a cameo?

Question

What are the "guide lines" to define an actor appearence as a *cameo*?Are there some specs to define it? For example, max time on screen, max scene, max lines...

Answer 1

It generally means that they have an unscripted appearance on set or that they are not credited in the movie. For instance, in Thor: Ragnarok, Matt Damon is seen briefly performing in a play depicting the events of an earlier movie and is not credited in the end credits for his role. This is a cameo.