Alternate Universe, or AU, refers to a story set in a different universe from the canon. This universe can be different in a few ways, such as AR (Alternate Reality) or AT (Alternate Timeline).
A story can be termed AU when the author makes major changes to the canonical storyline or premise, such as killing off a major character, changing characters’ motives or alliances, annulling major events or changing the setting.
- AR (alternate reality)
This is where the world is the same, but some (or most) basic canon facts are different. For example, an Emergency! where Dixie is the paramedic paired with Johnny and Kelly is an intern.
- AT (alternate timeline)
This refers to fan fictions that take place in another time than the canon. For example, Combat taking place during the Civil War instead of Word War II.
Canon refers to the “official” source material upon which fan fiction can be based, e.g., episodes of a series in which details are specifically shown or otherwise directly revealed.
The crossover story is one in which either characters from one story exist in (or are transported to) another pre-existing story’s world, or more commonly, characters from two or more stories interact.
Fanon is invented (non-canon or not verified as being canon) facts or situations, especially those which are used so frequently in fan fiction that they become seen by many as an extended part of the canon. For example, in Bonanza, it is fanon that Little Joe was born prematurely.
One of the usual purposes of fanon is to fill in perceived contradictions or gaps in the canon by answering (or asking) questions that the source material either will not or cannot address or simply hasn’t addressed before.
“Hurt/comfort” in a plot framework in which one character experiences pain (physical or emotional) and another character offers comfort.
An unusually powerful or favored-by-the-author character. This character may represent the author’s own wish-fulfillment fantasy. Often the Mary Sue character will rescue the nominal main character from a series of impossible situations, or nurse him back to health after he is seriously wounded or driven to the edge of insanity. The term can be used derisively by saying a story is contaminated by “Mary Sue-age” where the “Sue-age” is a homonym of “sewage.”
Original character refers to a character created by the author of the fan fiction, as opposed to one already existing in canon.
A story containing events that precede an existing series or episode. A prequel often explains the background which led to the events in the original. Sometimes, prequels play on the reader’s knowledge of what will happen next, using deliberate references to create dramatic irony.
A story containing events subsequent to an existing series or episode. A sequel may or may not concern the same plot as the work from which it is derived.
Fanfic writers often explore what might have happened if a certain canon episode had turned out differently. WHI – What Happened Instead (or In Between), WHN – What Happened Next, WHB – What Happened Before are all examples.