n.

A piece of fiction within a fandom utilizing characters and situations from a pre-existing work including (but not limited to) books, television programs, films, and comic strips.

Typically separated into het, slash, and general genres. Often used to play out AU scenarios and/or various romantic pairings not found in the original work.

Distributed via mailing lists, blogs, and zines. Heavily archived online.

I noticed most of the other definitions here completely trash fanfiction.

First, definition: Fanfiction is when someone takes either the story or characters (or both) of a certain piece of work, whether it be a novel, tv show, movie, etc, and create their own story based on it. Sometimes people will take characters from one movie and put them in another, which is called a cross-over.

Most people who bash fanfiction are not willing to look past the fact that it's based on something else to see that it could be worth reading IF you like the subject. If they don't like the subject, then obviously they are going to automatically dismiss the fanfiction.

It is true, however, that some fanfictions are rather poorly written and only a few hundred words, and it is also true that some people just write them so they can have their favorite characters have sex (lemon). But, if you take the time to find something decent, you can end up with a fanfiction story that is so close to the original piece of art, that you'd barely notice the difference.

Fiction written by fans for other fans about the fandoms that they like. Usually the author takes the characters and/or the setting of the fandom and writes a new or continueing plot. Sometimes they write crossovers

A work of fiction utilizing characters from a book, television show, or movie, as opposed to original characters. Referred to as "fan"fiction because it is generally written by fans obsessed with the book, television show, or movie.

1. Put bluntly, fiction by the fans. Created by fans of any particular fandom, 50% of fanfiction is crap, 24% is non-crappy smut, 25% fits into the category of crap and smut, and the remaining 1% is some pretty good stuff.
2. Something English teachers don't like.

Stories written by fans of a certain TV show, cartoon, anime, book, or movie using existing plots, characters, or ideas from the series but then continuing the story, adding new characters, changing the ending, or changing the plot.
Most are written by an obsessed fan who invents a character that's supposed to be like him/her, only 10 times prettier, smarter, and stronger who falls in love with whoever the fan has a crush on. Many of them have poor grammar, thin plots, and bad spelling. But there are a few with original ideas, great storylines, and interesting fancharacters writeen by creative writters. These ones are worth reading.
Most fanfiction is found only at fansites of the series or a general site for all types of fanfiction.

Something this English teacher heartily approves of as a way for writers to hone their art in a supportive community.
Some of the best stuff I've ever read can be found in the fandom!