An alternative universe (AU, also known as an alternate universe, alternative timeline, alternate timeline, alternative reality, or alternate reality) is the occurrence of canonical facts about the setting or characterization of a particular fictional universe being explored in a non-canonical way. These universes are often made unofficially, though elements of fan fiction are sometimes used in official, though usually non-canonical, storylines.

Stories that fall into this definition are usually "what-ifs", where possibilities arising from circumstances that do not occur in the original fictional universe are explored. For example, the premise of an alternative universe story may go something like this: "What would happen if character X was killed before event Y could happen?" Unlike regular fan fiction, which generally remains within the boundaries of the canon set out by the author, alternative universe fiction writers like to explore the possibilities of pivotal changes made to characters' history, motivations, or environment.