Fandom (from the noun fan and the affix -dom, as in kingdom, freedom, etc.) is a term used to refer to a subculture composed of fans characterized by a feeling of sympathy and camaraderie with others who share a common interest. A fandom can grow up centered around any area of human interest or activity. The subject of fan interest can be narrowly defined, focused on something like an individual celebrity; or wide, encompassing entire hobbies, genres or fashions. Fandom as a term can also be used in a broad sense to refer to an interconnected social network of individual fandoms, many of which overlap. While it is now used to apply to groups of people fascinated with any subject, the term has its roots in those with an enthusiastic appreciation for sports. The Webster's Dictionary traces the usage of the term back as far as 1903.

Fans typically are interested in even minor details of the object(s) of their fandom and spend a significant portion of their time and energy involved with their interest; this is what differentiates them from those with only a casual interest.

When referring to an organized sub-culture, the term "fandom" is most often associated with science fiction fandom, the community of fans of the science fiction and fantasy genres, an international fan sub-culture which dates back to the 1930s and has held the annual World Science Fiction Convention since 1939 along with many other events each year. Science fiction fandom maintains organized clubs and associations in many cities around the world, and has created its own jargon, sometimes called "fanspeak". Many other fandoms have their origins in science fiction fandom (and often still overlap with it), and derive some of their jargon, customs and practices from those roots.

Members of a fandom associate with one another, often attending fan conventions (such as science fiction conventions), and publishing and exchanging fanzines. Originally using print-based media, these sub-cultures have migrated much of their communications and interaction onto the internet, which they also use for the purpose of archiving detailed information pertinent to their given fanbase. Some fans also write fan fiction, stories based around the universe and characters of their chosen fandom. Some also dress in costumes ("cosplay") or recite lines of dialogue either out-of-context or as part of a group reenactment. Such activities are sometimes known as "fanac," an abbreviated form of the phrase "fan activity." The advent of the internet has significantly facilitated fan association and activities.

The entertainment industry refers to the totality of fans devoted to a particular area of interest, whether organized or not, as the "fanbase".

Fans, have, on occasion, organized on behalf of cancelled television series, with notable success in cases such as Star Trek in 1968, Cagney & Lacey in 1983, Roswell in 2000 and 2001 (it was cancelled with finality at the end of the 2002 season), Farscape in 2002, Firefly in 2002, and Jericho in 2007. (In the case of Firefly the result was a movie, not another season.) Such outcry, even when unsuccessful, suggest a growing self-consciousness on the part of entertainment consumers, who appear increasingly likely to attempt to assert their power as a bloc. Fan activism in support of the 2007 Writers Guild of America strike through Fans4Writers appears to be an extension of this trend.

Fandom is sometimes caricatured as religious faith, as the interest of fans sometimes grows to dominate their lifestyle. However, society at large does not treat fandom with the same weight as organized religion, and fans themselves often become divided by the strength of their own belief.

Feature-length documentaries about fandom (some more respectful of the subjects than others) include Trekkies (film), Finding the Future: A Science Fiction Conversation, and Done the Impossible. "Fandom" is also the name of a documentary / mockumentary about a fan obsessed with Natalie Portman.