The community that surrounds a tv show/movie/book etc. Fanfiction writers, artists, poets, and cosplayers are all members of that fandom. Fandoms often consist of message boards, livejournal communities, and people.

Fan Domain
A domain in the internet about a comic, a book, a tv show, a video game etc. There are several fandoms all around the internet, where fans converge and the comunities are really big, many have forums, fanfics, fanart, etc.

The microcosm made up of people who are fans of a fiction genre, or of a subgenre, who have their own clubs, conventions and amateur magazines (fanzines), dating back (in some cases) to the early 1930s. Some of these are overlapping. Star Trek fandom, for example, started in science fiction fandom and eventually became a separate fandom on its own. Comics fandom and even rock (music) fandom also started there before becoming separate entities.

In fanfiction; the movie, TV show, book or other medium that a fanfiction is written about.

the way a fan shows his dedication to whatever he's a fan of

In terms of any literary/visual media, be it TV show, movie, book, play, cartoon, or comic book. The characters, plots, and settings therein.

Fucking random.