At one point, on the old FanFiction Mailing List (a listserv for anime fanfiction I was active on -- moderator of, in fact, for about a year -- from the mid-nineties until about 2004ish, when the then-owner of the FFML server farm passed away, and the server was disabled without anyone to pay the internet service fees; I believe it still continues in some form, somewhere, but I haven't been a part of it since), we had a discussion of various different types of AU, and whether these would be "better" descriptors for indicating what type of a fic it is:
"Divergence" (everything is canon up to a certain point, after which it's anything goes)
"Butterfly Effect" (known on the TVTropes as "For Want of a Nail" type fics) -- one minor change is made to the story early on, and the consequences of that change. (a Naruto fic I was working on, but scrapped and cannibalized for other things, would have been "what if Kakashi had injured his leg and couldn't give Team Seven the Bell Test in part one?" Although my "nail\butterfly wing flap" was actually earlier, effecting Tenten (who, in turn, accidentally caused the injury to Kakashi)
"Gender-bending" (a partial or complete retelling of the story based on the principle that one or more major characters in the series took on the opposite gender and, hence, opposite gender roles)
"Fusion" (generally associated with Crossovers, the term was (from what I heard) coined because it fused elements of crossover with elements of AU. Fusion is where the characters from one series are placed into another series (or a real life event) replacing the roles of the other characters; while this if often attempted, it is rarely done WELL, and the only way it can be done well is if the characters are all completely in character. Related is the Crossfusion, which I've only seen done once with any degree of quality (A Faint Hope, by Lara Bartram et al, which was a crossfusion of Ranma and Star Wars), where the characters from one series appear to take on the roles of the characters from another series in that other series universe, but the original characters are in fact there, fulfilling canon of that universe, and the "heroes" of the new series -- while still largely (or completely) in character for their own -- are having their own adventure parallel to that actual universe heroes, only occasionally running into the original universe heroes during "off camera" moments (often explaining things like "Why did Obi-wan just sacrifice himself like that?" Answer: He was running away from back alimony payments owed to his ex-wife, Nodoka Kenobi, who he just happened to spot out of the corner of his eye during his fight with Vader). Complicated, I know, but when it works right it is absolutely brilliant... and hilarious, because you have to do this kind of thing as a comedy)
"Elseworld" (this is the "Inu Yusha is a Rock Star, Kagome is his groupie" type mentioned above; the subset of "High School Fics" fits largely in this catagory. The name given comes from comic books and is often used as a synonym to AU, but when we were discussing this it seemed appropriate. It CAN be done well, though no specific examples come to mind, but usually isn't)
Edited by desaix, 09 April 2012 - 04:14 PM.