A shared universe or shared world is a set of creative works where more than one writer (or other artist) independently contributes a work that can stand alone but fits into the joint development of the storyline, characters, or world of the overall project.
The term shared universe is also used within comics to reflect the overall milieu created by the comic book publisher in which characters, events, and premises from one product line appear in other product lines in a media franchise.
Fiction in some media, such as most television programs and many comic book titles, is understood to require the contribution of multiple authors and does not by itself create a shared universe and is considered a collaborative art form. Incidental appearances, such as that of d'Artagnan in Cyrano de Bergerac, are considered literary cameo appearances. More substantial interaction between characters from different sources is often marketed as a crossover. While crossovers occur in a shared universe, not all crossovers are intended to merge their settings' back-stories and are instead used for marketing, parody, or to explore "what-if" scenarios.
It can become difficult for writers contributing to a shared universe to maintain consistency and avoid contradicting details in earlier works, especially when a shared universe grows to be very large. The version deemed "official" by the author or company controlling the setting is known as canon. Not all shared universes have a controlling entity capable of or interested in determining canonicity, and not all fans agree with these determinations when they occur. A fanon may instead find some degree of consensus within the setting's fandom.
Some writers, in an effort to ensure that a canon can be established and to keep details of the setting believable, employ tools to correct contradictions and errors that result from multiple contributors working over a long period of time. One such tool is retconning, short for "retroactive continuity", which resolves errors in continuity that came about through previously-written conflicting material.
Readers may also object when a story or series is integrated into a shared universe, feeling it "requir[es] one hero's fans to buy other heroes' titles".
Originating in novels
The expansion of existing material into a shared universe is not restricted to settings licensed from movies and television. For example, Larry Niven opened his Known Space setting to other writers initially because he considered his lack of military experience to prevent him from adequately describing the wars between mankind and the Kzinti. The degree to which he has made the setting available for other writers became a topic of controversy, when Elf Sternberg created an erotic short story set in Known Space following an author's note from Niven indicating that "[i]f you want more Known Space stories, you'll have to write them yourself". Niven has since clarified that his setting is still to be used only "under restricted circumstances and with permission", which Niven granted to the several authors of the Man-Kzin Wars series. By contrast, author Eric Flint has edited and published collaborations with fan fiction writers directly, expanding his 1632 series.
A setting may also be expanded in a similar manner after the death of its creator, although this posthumous expansion does not meet some strict definitions of a shared universe. One such example is August Derleth's development of the Cthulhu Mythos from the writings of H. P. Lovecraft, an approach whose result is considered by some to be "completely dissimilar" to Lovecraft's own works. Less controversial posthumous expansions include Ruth Plumly Thompson's and later authors' sequels to L. Frank Baum's Oz stories and the further development of Isaac Asimov's Foundation universe by Greg Bear, Gregory Benford, and David Brin.
Many other published works of this nature take the form of a series of short-story anthologies with occasional standalone novels. Examples include Robert Lynn Asprin's Thieves' World, C. J. Cherryh's Merovingen Nights and Janet Morris' Heroes in Hell.
Originating in comics
By 1961, Marvel Comics writer and editor Stan Lee, working with artists Jack Kirby and Steve Ditko, merged the bulk of the publisher's comics characters into the Marvel Universe. Marvel sets its stories in an increasing number of alternate realities, each with an assigned number in a greater "multiverse". DC and Marvel have also periodically co-published series in which their respective characters meet and interact. These intercompany crossovers have typically been written as self-limiting events that avoid implying that the DC Universe and Marvel Universe co-exist. Exceptions include the twenty-four comics released under the metafictional imprint Amalgam Comics in 1996, depicting a shared universe populated by hybridizations of the two companies' characters. Marvel has since referred to this as part of its setting's greater multiverse by labeling it Earth-692.
Although DC and Marvel's shared universe approaches to comics have set them apart from competitors in the industry, other companies have attempted similar models. Valiant Comics and Crossgen both produced titles primarily set from their inception in a single, publisher-wide shared universe, known respectively as Unity and the Sigilverse.
Originating in film and television
The Star Wars franchise introduced a four-tier system based on compatibility with the original six films. Star Trek canon is less well-defined, generally excluding not only licensed works such as books and video games and acknowledging that "even events in some of the movies have been called into question". Both franchises have blurred the lines between canon and non-canon content by adopting unofficial material into later official productions. 
The spin-off media extending of the universe originating in Doctor Who has relatively little consistency given its division into audio plays produced by Big Finish and the BBC, the New Adventures universe novel, or a universe based on comics published in Doctor Who Magazine and other publications.
Originating on the internet
The influence of the Internet on collaborative and interactive fiction has also resulted in a large number of amateur shared universe settings. Amateur authors have created shared universes by contributing to mailing lists, story archives and Usenet. One of the earliest of these settings, SFStory, saw its spin-off setting Superguy cited as illustrative of the potential of the Internet. Another example is the furry-themed Tales from the Blind Pig created at the Transformation Story Archive which some limited publication. Other early examples include the Dargon Project and Devilbunnies.
- 2008 The Universal Monsters cinematic universe (2014–present) 
- Doctor Who, Torchwood and The Sarah Jane Adventures 
- Thieves' World, originally created and edited by Robert Lynn Asprin and Lynn Abbey
- Heroes in Hell, edited by Janet Morris
- Trillium series by Marion Zimmer Bradley, Andre Norton and Julian May.
- Buffyverse (Buffy the Vampire Slayer and Angel and related media) 
- 2003 - 2012 One Tree Hill and Life Unexpected
- 2011 American Horror Story (all seasons share the same timeline)
- Marvel Cinematic Universe (films and short films and TV series and related media) 
- 2000 X-Men and Fantastic Four (2015)
- 2007 Transformers
- 2013 DC Extended Universe
- 2013 Justice League: The Flashpoint Paradox starts a universe based on the New 52 that includes various Justice League and Batman films. 
- Expanded universe
- Fictional crossover
- Fictional universe
- Media franchise
- Setting (narrative)
- Spin-off and sister show
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