From Media Technology and Culture Change

Fandom is a noun born of a combination of fan with the affix -dom (i.e., kingdom). The affix implies that fandom is a (non-physical [though fandom can and does have physical manifestations in art, conventions, etc.]) realm for the fans of a given media work.

The fandom is generally an extension to the universe explored and created in the original book(s), television shows, and/or film(s).

Fandom and Web 2.0

Most fandoms were early adopters of the ease in communication offered by Web 2.0. Many have built thriving online communities, particularly with the aid of forums.

Fandom and Meta-media objects

Examples of Extensive Fandoms