An anti-fan is someone who enjoys writing, discussing or in some cases making derivative works about a piece of media, but solely for the purpose of railing against or parodying it.
Anti-fans usually specify the fandom they are opposed to within their name, e.g. Anti-Twilight fans. It is common for large anti-fandoms to gather in groups, usually on forums and sites, to share their mutual aversion. These are coined anti-fan clubs, and some are substantial enough to become anti-fan sites.
While fan studies, as part of media studies, have been given significant attention, little research has been done for those who engage just as obsessively with media that they claim to oppose. Academics Henry Jenkins and Matt Hills have done some research on the phenomenon of anti-fans.
A sports fan dedicated to cheering against a specific team can be considered an anti-fan as well. An "Anti-Fan" is said to cheer for whomever the opponent of their team is playing. In the United States, this happens often in college sports, as most colleges have defined or traditional rivals. This concept was noted in a set of ESPN commercials tagged "Never Graduate" and "Long Live Rivalry", where graduates of popular colleges interact with graduates of rival schools in humorous agitating ways.