Fan fiction or fanfiction, also known as fan fic, fanfic, fic or FF, is fiction typically written in an amateur capacity by fans as a form of fan labor, unauthorized by, but based on, an existing work of fiction. The author uses copyrighted characters, settings, or other intellectual properties from the original creator(s) as a basis for their writing and can retain the original characters and settings, add their own, or both. Fan fiction ranges in length from a few sentences to novel-length and can be based on fictional and non-fictional media, including novels, movies, comics, television shows, musical groups, cartoons, anime and manga, and video games.

Fan fiction is rarely commissioned or authorized by the original work's creator or publisher or professionally published. It may infringe on the original author's copyright, depending on the jurisdiction and on legal questions, such as whether or not it qualifies as "fair use" (see Legal issues with fan fiction). The attitudes of authors and copyright owners of original works towards fan fiction have ranged from encouragement to indifference or disapproval, and they have occasionally responded with legal action.

The term came into use in the 20th century as copyright laws began to distinguish between stories using established characters that were authorized by the copyright holder and those that were not.

Fan fiction is defined by being related to its subject's canonical fictional universe, either staying within those boundaries but not being part of the canon, or being set in an alternative universe. Thus, what is considered "fanon" is separate from canon. Fan fiction is often written and published among fans, and as such does not usually cater to readers without knowledge of the original media.

The term fan fiction has been used in print as early as 1938; in the earliest known citations, it refers to amateur-written science fiction, as opposed to "pro fiction". The term also appears in the 1944 Fancyclopedia, an encyclopaedia of fandom jargon, in which it is defined as "fiction about fans, or sometimes about pros, and occasionally bringing in some famous characters from [science fiction] stories". It also mentions that the term is "sometimes improperly used to mean fan science fiction; that is, ordinary fantasy published in a fan magazine".

Before the adoption of copyright in the modern sense, it was common for authors to copy characters or plots from other works. For instance, Shakespeare's plays Romeo and Juliet, Much Ado About Nothing, Othello, As You Like It and The Winter's Tale were based on recent works by other authors of the time.

In 1614, Alonso Fernández de Avellaneda wrote a sequel to Miguel de Cervantes's Don Quixote before he had finished and published his own second volume.

There is a debate within the fanfiction community as to whether or not Dante Alighieri's Inferno can be considered self-insert fanfiction of the Bible.^{[citation needed]}