An alternative universe (also known as alternate universe or alternate reality), commonly abbreviated as AU, is a type or form of in which canonical facts of setting or characterization in the universe being explored or written about are deliberately changed.

Stories that fall into this definition are usually what-ifs, where possibilities arising from different circumstances or character decisions are explored. 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.