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
I once read a Xena, Warrior Princess/Star Treck: Voyager uber, crossover, fanfic.
In terms of any literary/visual media, be it TV show, movie, book, play, cartoon, or comic book. The characters, plots, and settings therein.
The X-men Fandom is made up by its original comic books and original cartoons as well as the new X-men:generation movies and cartoons.