Fan fiction (alternately referred to as fanfiction, fanfic, FF, or fic) is a broadly-defined term used to describe stories about characters or settings written by fans of the original work, rather than by the original creator. Works of fan fiction are rarely commissioned or authorized by the original work's owner, creator, or publisher; also, they are almost never professionally published. Fan fiction, therefore, is defined by being both related to its subject's canonical fictional universe and simultaneously existing outside the canon of that universe. Most fan fiction writers assume that their work is read primarily by other fans, and therefore tend to presume that their readers have knowledge of the canon universe (created by a professional writer) in which their works are based.
Before about 1965, the term "fan fiction" was used in science fiction fandom to designate original, though amateur, works of science fiction published in science fiction fanzines, as differentiated from fiction that was professionally published by professional writers; or fiction about fans and fandom; but this usage is now obsolete. Modern definitions of the term exclude such entirely original writing from the category. Today, "fan fiction" writers are those who use characters and situations already created by other writers in order to develop their personal and preferred views of the story. For example, the Harry Potter series by J.K. Rowling has spawned an extensive fan fiction subgenre, in which the characters make choices and enjoy an afterlife that Rowling herself did not create, or deviate from the plotline of the actual books at a certain point, allowing a different route to be traveled.
The magazine, Spockanalia, is the first known contemporary form of any type of fan-written stories.

Fan fiction as it is now understood began at least as early as the 17th century, with unauthorized published sequels to such works as Don Quixote. Older precursors include the Epic Cycle supplementing the works of Homer and the various re-tellings of King Arthur's tale which spread around Europe from the 8th century AD onward. (For example, there were no fewer than four continuations by other hands of Chrétien de Troyes' Perceval.) Another precursor is in medieval Arabic fiction, where story cycles were formed as different storytellers added sequels to an original story, such as story cycles revolving around Harun al-Rashid or Sinbad the Sailor. Some tales were also parodied by other storytellers. Many of these story cycles and parodies were incorporated into the Arabian Nights. For example, "Wardan the Butcher's Adventure With the Lady and the Bear" is parodied by "The King's Daughter and the Ape", "Harun al-Rashid and the Two Slave-Girls" is parodied by "Harun al-Rashid and the Three Slave-Girls", and "The Angel of Death With the Proud King and the Devout Man" is parodied by "The Angel of Death and the Rich King". Sometimes even parodies were parodied, such as "The Angel of Death and the King of the Children of Israel" being a parody of "The Angel of Death and the Rich King". When the Arabian Nights was translated into French at the beginning of the 18th century, many imitations of it were written in France.

The turn of the 19th century saw parodies and revisions of Lewis Carroll's Alice in Wonderland by authors including Frances Hodgson Burnett and E. Nesbit. In addition, there were several fan-authored versions of Arthur Conan Doyle's Sherlock Holmes stories. In the 1920s and 1930s, fans of Jane Austen wrote stories based on her characters and published them in fanzines. In 1945, C. S. Lewis adopted certain elements from J. R. R. Tolkien's then largely unpublished legendarium (mostly Númenor, there spelt Numinor) and incorporated these into the last novel, That Hideous Strength, of his Space Trilogy. (Given that Lewis and Tolkien were personal friends, this could be seen more as an "homage").

However, the modern phenomenon of fan fiction as an expression of fandom and fan interaction was popularized and defined via the Star Trek fandom and fanzines published in the 1960s. The first Star Trek fanzine, Spockanalia, was published in 1967 and contained some fan fiction. In 1970, Mary Ellen Curtin, a Princeton University graduate, researched the authors of various Star Trek fanfics, and her results showed an outstanding 83% female as opposed to 17% male writers.

A similar trend in Japan also began appearing around the 1960s and 1970s, where fan fiction is known as dōjin or dōjinshi, which mostly revolve around manga and anime. Manga authors like Shotaro Ishinomori and Fujiko Fujio formed dōjin groups such as Fujio's New Manga Party (新漫画党, Shin Manga-to?). At this time dōjin groups were used by artists to make a professional debut. This changed in the coming decades with dōjin groups forming as school clubs and the like. This culminated in 1975 with the Comiket in Tokyo.

Both Listproc and Usenet were invented in 1980, allowing public Internet-based gatherings of fans, and wider distribution of fan fiction; the internet as a whole would later become the most widely-used method of publication of fan fiction.

In 1981 Lucasfilms Ltd. sent out a letter to several fanzine publishers asserting Lucasfilm's copyright to all Star Wars characters and insisting that no fanzine publish pornography. The letter also alluded to possible legal action that could be taken against fanzines that did not comply. Later that year, the director and legal counsel of the Official Star Wars Fan Club sent fanzine publishers a set of official guidelines. Lucasfilms supported fan publications contingent on their upholding these guidelines.

The Gopher protocol was invented in 1991, and hosted some early fan fiction archives, but it has since been replaced by the World Wide Web, which was created a year later.

Fan fiction has become more popular and widespread since the advent of the World Wide Web. Many archives such as The Gossamer Project, which focuses primarily on X-Files fanfiction, were created, hosting specific sorts of stories, or stories for specific fandoms. In 1998 FanFiction.Net came online. At the time of its initial creation, it accepted any sort of writing, original or fan fiction. It has since separated its original fiction section to another website and banned several subgenres, including explicitly sexual stories (referred to as "NC-17" before the Motion Picture Association of America chose to enforce its ownership of the MPAA ratings system), real person fiction, and stories featuring song lyrics (the latter two in order to avoid legal problems, including copyright infringement for unauthorized use of lyrics). This ability to self-publish fan fiction at a common archive, and the ability to review the stories directly on the site, became popular quite quickly.
As of 2005, teenagers born between the years 1985 and 1992 made up the majority of authors on one fanfiction site

FanFiction.net now hosts millions of stories in dozens of languages, and is widely considered the largest and most popular fan fiction archive online. LiveJournal (founded in 1999) and other blogging services played a large part in the move away from mailing lists to blogs as a means for fan communication and the sharing of fan fiction; although much fan fiction today is published to archives, it would be impossible to tell whether more or less fan fiction today is posted directly to blogging services than to fan-fiction-specific archives.