Something that is believed to be true by the person who is telling a story, but in reality it's an outright fabrication.
Boy 1: Hey, Have you seen Mean Girls 3?
Girl 1: There isn't one.
Boy 2: Are you sure that wasn't a dream?
Boy 1: No! There's a Mean Girls 3!
Girl 2: I think this a bit of a The Umbrellas of Cherbourg moment.