An alternative universe is a type or form of fan fiction in which known, canonical facts about the universe being explored or written about, are deliberately changed. In Alternative Universe stories, characters' known motivations may vary considerably from their actual decisions. An "alternative universe" example from English Literature involving Jane Austen's Pride and Prejudice characters might have Elizabeth not marry Mr. Darcy. The author of an alternative universe story thus can use the same characters, but send them down different paths in a different plot.

Commonly abbreviated AU, stories of this type are usually "what-ifs", where possibilities arising from different circumstances, decisions, etc., are explored. The appeal of the AU for both writer and reader is the exploration of which facets of the characters we know were/are determined by their environments. (What if Luke Skywalker grew up with his sister on Alderaan?) In addition, the author also gets a built in audience for their story based on the fans for that universe which they would not get if they they wrote it as an original story instead of fan fiction.

Alternative universe settings are also written when the author was unhappy with the direction the original source took, a major example being The Gargoyles Saga fan fiction series, which ignores all but one of the episodes of Gargoyles' third and final season. The are also known as fix-its , HEXes or denial-fics . Stories in this category of AU follow the established canon before veering away a a crucial moment.

Also within this arena is the idea of taking all the characters from the series, placing them in a different time/place/situation, and working with it as a new series. An example of this is "Simple and Clean," by Ethelflaed [1] a Yu-Gi-Oh Anime AU fic set in 1066 in Normandy. Although this technique has been tried in the actual canon of many fandoms, it is still debated as a valid fanfiction form.

The central question of an AU is 'how far are we determined by our surroundings?'