An alternate universe (also known as alternative 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.
The author gets an established audience for their story: the fans of the original, and then subsequently altered, universe, which they would not get if they wrote it as an original story instead of fan fiction. Some of the best fan fiction writers, who aspire to be published authors, can take advantage of this inversely by developing a loyal audience, the readers of their fan fiction, for books with original storylines that they might publish at a later date.