Fandom (from the noun fan and the affix -dom, as in kingdom, dukedom, etc.) refers to a community of keen aficionados who share a common interest in any phenomenon, such as authors, hobbies, genres or fashions. Fandom as a term can also be use to refer to the single interconnected network of these individual fandoms, many of which overlap.

Fans (or the plural fen) typically are interested in even minor details of the object of their fandom; this is what differentiates them from those with only casual interest.

The objects of a fandom typically relate to the arts, sports or entertainment. For example, it would be unusual to refer to an accountant who is very interested in the details of accounting as a “fan” of accounting.

Members of a fandom associate with one another, often attending fan conventions (such as science fiction conventions), and publishing and exchanging fanzines. Today, these communities are often online, especially for less well-known source material.

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.

The term “fandom” is particularly associated with fans of the science fiction and fantasy genres, a community that dates back to the 1930s and has held the World Science Fiction Convention since 1939. The Oxford English Dictionary traces the usage of the term back as far as 1903, with many of its documented references referring to sports fandom.

The term is also commonly associated with anime/manga. Serious fans of this subject are also called otaku.

“Fandom” is also the name of a documentary / mockumentary about a fan obsessed with Natalie Portman. (via Wikipedia)

fandom (n.) antedating 1896 Washington Post, Oct 10, 1896, the world of enthusiasts for some amusement or for some artist; also in extended use. (via Science Fiction Citations)

The earliest known usage of the term “science fiction” is in 1851 (in Chapter 10 of William Wilson’s A Little Earnest Book upon a Great Old Subject), in which he writes: “Science-Fiction, in which the revealed truths of Science may be given interwoven with a pleasing story which may itself be poetical and true.”

However this appears to be an isolated usage and the term appears to have been recoined in the 1920s where it appeared in Amazing Stories.

Science fiction is often abbreviated as SF or sci-fi. However, SF is not unambigous (see under Other types, below), and sci-fi is seen by some as having derogatory overtones. A short lived synonym was scientifiction. (via Wikipedia)