A shared universe is a fictional universe to which more than one writer contributes. Works set in a shared universe share characters and other elements with varying degrees of consistency. Shared universes are contrasted with collaborative writing, in which multiple authors work on a single story. Shared universes are more common in fantasy and science fiction than in other genres. Examples include Star Wars, DC Universe, Marvel Universe, Star Trek, Forgotten Realms, Babylon 5, Foundation series, Dragonlance, Power Rangers, Man-Kzin Wars, Cthulhu Mythos and the 1632 series.
There is no formalized definition of when the appearance of fictional characters in another author's work constitutes a shared universe. 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. Incidental appearances, such as that of d'Artagnan in Cyrano de Bergerac, may instead be 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.
The modern definition of copyright, especially under United States copyright law, considers the expansion of a previous work's setting or characters to be a derivative work. Especially for material being considered for publication, this often necessitates licensing agreements. For this reason, some fan fiction and other amateur works written in established settings without permission, are sometimes distinguished from shared universe writings or even described as a "stolen universe". However, fair use claims have been raised, and not all authors believe that fan fiction should be distinguished from other literature in this manner at all. In a process similar to brand licensing, the intellectual property owners of established fictional settings at times allow others to author new material, creating an expanded universe. Such franchises, generally based on television programs or film, allow for series of novels, video games, original sound recordings and other media. Not all shared universe settings are simply the expansion or combination of pre-existing material by new authors. At times, an author or group of authors has created a setting specifically for development by multiple authors, often through collaboration.
Especially when a shared universe grows to include a large number of works, it becomes difficult for writers to maintain an internally consistent continuity and to avoid contradicting details in earlier works. The version that is 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 willing to determine 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", where later adjustments result in the invalidation of previously-written material. The most severe form of retcon involves a wholesale rewrite of the groundwork for the entire setting. These reboots, most closely associated with DC Comics, are not always effective at resolving underlying problems and may meet with a negative reaction from fans. Contributors to expanded universes, also known as tie-in writers, have sometimes been stereotyped as "hacks" because such writing is perceived as less creative or of consistently poor quality. These stereotypes have been disputed by authors who consider contributing to a larger setting "intellectually demanding."
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" or leads to mischaracterizations and inappropriate comparisons.
Expansion of existing material 
In 1941, writer Gardner Fox at All-American Comics (later part of DC Comics) created the Justice Society of America in All-Star Comics #3, credited with being the first superhero team-up and laying the groundwork for the DC Universe, the first comic book shared universe. 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. Both settings have suffered from the creative difficulties of maintaining a complex shared universe handled by large numbers of writers and editors. DC has substantially altered its in-universe chronology several times, in series such as Crisis on Infinite Earths in 1985, Zero Hour in 1994, and Infinite Crisis in 2005. As of 2007, Marvel has rebooted its continuity only once, in Spider-Man: One More Day. They instead set 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.
The Star Wars franchise takes a unique view regarding the canon properties of its expanded universe, introducing a four-tier system based on compatibility with the 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". Furthermore, both franchises have blurred the lines between canon and non-canon content by adopting unofficial material into later official productions.  The spin-off media making up an extension of the universe originating in Doctor Who is particularly complex due to the permissive stance on licensing and canon taken by the BBC. This Expanded Universe 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. Reviewer Robert F.W. Smith attempted to summarize the conflicting continuities:
As far as I understand it, the situation is this: the New Adventures universe is inside the bottle universe seen in Interference, which was built by BBC universe Time Lords, and in it, the NA Time Lords are all gone – they’ve gone to another bottle and left the NA universe to the Gods/Kings of Space. Most of the New Adventures happened in the BBC universe anyway, except in that universe, the 7th Doctor was the reincarnation of the Other and Rassilon escaped to roam the universe – in the BBC universe, he may or may not have been, and Rassilon probably didn’t. In the BBC universe, Faction Paradox, the Doctor and the Enemy between them have vaped the Time Lords, with the result that there are no longer any Time Lords in the BBC universe, except for five, the fundamental laws of the universe (the magic-and-science thing) have changed, and the Doctor is no longer a Time Lord at all originally but a crystal man named Soul from the end of time. Also in the BBC universe, an infinity of different universes have been released, which helpfully explains how all the shock companion-killings in the novels ever since Eternity Weeps either did or didn’t happen in our universe, according to whatever criteria you like, but Gallifrey didn’t survive in any of them. Despite this, Gallifrey will still be rebuilt in the BBC universe in some form, but it will presumably be much less powerful because it will now be a planet without the original’s special relationship with time, and it won’t have always been there. Where the Big Finish audios fit in is anybody’s guess; the new series can just about be assumed to follow on from the end of The Gallifrey Chronicles, even though the Doc says he’s a Time Lord – not a crystal man from the end of time – in the second episode. There.
Even Smith's summary does not address spin-offs such as the Bernice Summerfield novels and the Faction Paradox series that are legally distinct from the origins of their characters in officially licensed novels. Many fans consider only the television series canon and all other media apocrypha. The television series has never explicitly acknowledged any of the spin-offs, partly because the BBC's status as a public service broadcaster prohibits them from producing a programme that can only be fully understood by those who have purchased licensed products.
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.
Original settings 
Although DC and Marvel's successful shared universe approaches to comics have set them apart from competitors in the industry, other companies 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.
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.
Role-playing games are inherently designed to include some aspects of the shared universe concept, as individual games are derived from the core material. Campaign settings, such as Dungeons & Dragons's Faerûn, Dragonlance and Eberron, provide a more detailed world in which novels and other related media are additionally set. Living campaigns, including the RPGA's Living Greyhawk or the AEG-sanctioned Heroes of Rokugan, provide an opportunity for individual games hosted worldwide to take part in a single continuity.
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 differs from many amateur settings both by having an organized effort to maintain consistent canon and by having seen at least limited publication. Other early examples include the Dargon Project and Devilbunnies.
Animated universes 
An animated universe is a derivative of the shared universe that applies to animated works, including animated television series and films, which share the same characters and continuity with each other; it may also derive from the characters and continuity of previous literary serial works, including comics, and may thus be termed by the literary work from whence it derived. Notable examples of the animated universe include:
- Masters of the Universe which included He-Man and She-Ra in the 80s
- DC animated universe (1992–2006)
- Marvel animated universe that included crossovers between Spider-Man and X-Men in the mid-90s
- Family Guy, American Dad and Cleveland Show (1999-present)
Anime universes 
Japan has also created many animated universes:
- Akira Toriyama's various serious including Dr. Slump, Dragon Ball and other one-shot Manga (1979-2007)
- Gundam "metaseries" (1979–present)
- Mazinger has crossed over with other robot fighter animes such as Devilman, Getter Robo and Grendizer
- Scramble Wars universe included AD Police, Bubblegum Crisis, Gall Force and Riding Bean
- Super Dimension series
- Brave Series of the 90s particularly had GaoGaiGar spawning Betterman in the same universe
- Gokinjo Monogatari and its spinoff Paradise Kiss
- Outlaw Star and Angel Links (1998-1999)
- Cicadas Cry and Seagulls Cry have a character in common
- Fate/stay and Tsukihime have a character in common
- CLAMP's series including Cardcaptor Sakura, Tsubasa Chronicle and xxxHolic among others
- Dijiko and Puchiko of Di Gi Charat are also characters in Cromartie High School (episode 25)
Live-action TV universes 
- The Star Trek franchise, including the original series, Star Trek: The Next Generation, Star Trek: Deep Space Nine, Star Trek: Voyager, and Star Trek: Enterprise (as well as the many theatrical movies generated by the franchise).
- Magnum, P.I., Simon & Simon, Whiz Kids and Murder, She Wrote:
- A two-part crossover between Magnum, P.I. and Simon & Simon has the Simon brothers of the latter appear on the former and Magnum and Higgins of the former appear on the latter.
- A.J. Simon of Simon & Simon appears on the Whiz Kids episode "Deadly Access".
- Several of the Whiz Kids characters appear on the Simon & Simon episode "Fly the Alibi Skies".
- A two-part crossover between Magnum, P.I. and Murder, She Wrote has Jessica Fletcher of the latter appear on the former and Magnum and Higgins of the former appear on the latter.
- JAG, First Monday, NCIS, NCIS: Los Angeles, Hawaii Five-0 and NCIS: Red:
- Edward Sheffield of First Monday became a recurring character on JAG after the cancellation of the former.
- The JAG episodes "Ice Queen" and "Meltdown" serve as the pilot for NCIS.
- Bud Roberts of JAG appears on the NCIS episode "Hung Out to Dry".
- Captain Thomas Lind of JAG appears on the NCIS episode "A Man Walks Into a Bar...".
- Lt. Commander Faith Coleman from the JAG episode "Meltdown", appears on three episodes of NCIS.
- Commander Michael Rainer from the JAG episode "Good Intentions" appears on the NCIS episode "A Weak Link".
- The two-part NCIS episode "Legend" serves as the pilot for NCIS: Los Angeles.
- Leon Vance of NCIS appears on nine episodes of NCIS: Los Angeles.
- Abby Sciuto of NCIS appears on two episodes of NCIS: Los Angeles.
- Lee Wuan Kai from the NCIS: Los Angeles episode "Killshot" appears on the NCIS episode "Endgame".
- Trent Kort of NCIS appears on the NCIS: Los Angeles episode "Callen, G".
- Kensi Blye of NCIS: Los Angeles appears on the Hawaii Five-0 episode "Ka Hakaka Maika'i".
- A two-part crossover between Hawaii Five-0 and NCIS: Los Angeles has Sam Hanna and G. Callen of the latter appearing on the former and Danny Williams and Chin Ho Kelly of the former appearing on the latter.
- CSI: Crime Scene Investigation, CSI: Miami, Without a Trace, Cold Case and CSI: NY:
- The CSI episode "Cross-Jurisdictions" serves as the pilot for CSI: Miami.
- The CSI: Miami episode "MIA/NYC Nonstop" serves as the pilot for CSI: NY.
- A two-part crossover between CSI: Miami and CSI: NY has Mac Taylor of the latter appearing on the former and Horatio Caine of the former appearing on the latter.
- Detective Scotty Valens of Cold Case appears on the CSI: NY episode "Cold Reveal".
- A two-part crossover between CSI and Without a Trace has Jack Malone of the latter appearing on the former and Gil Grissom of the former appearing on the latter.
- A three-part crossover between the three CSI series known as the CSI Trilogy features Dr. Raymond Langston of the original CSI.
- A two-part crossover between CSI and CSI: NY has been announced that will feature Mac Taylor of the latter appearing on the former and D.B. Russell of the former appearing on the latter.
- Eureka, Warehouse 13 and Alphas:
- Douglas Fargo of Eureka appears on two episodes of Warehouse 13.
- Claudia Donovan of Warehouse 13 appears on the Eureka episode "Crossing Over".
- Dr. Vanessa Calder from four episodes of Warehouse 13, appears on the Alphas episode "Never Let Me Go".
- Third Watch, ER and Medical Investigation.
- A two-part crossover between Third Watch and ER has Maurice Boscorelli, Faith Yokas, Kim Zambrano, Alex Taylor and Lt. Swersky of the former appear on the latter and Susan Lewis of the latter appearing on the former.
- A two-part crossover between Medical Investigation and Third Watch has Drs. Stephen Connor and Nathalie Durant of the former appear on the latterand Faith Yokas, Carlos Nieto and Levine of the latter appear on the former.
- Both Buffy the Vampire Slayer and Angel are set in the Buffyverse
- Angel was originally a character on Buffy. Spike and Cordelia from Buffy also star in Angel.
- Stargate: Stargate: SG-1, Stargate Atlantis, Stargate Universe
- SG-1 was the follow-up to the film which stayed focused on Earth. Atlantis was about a team that travelled through the Stargate to the homeworld of the Gate's makers. Universe is the newest.
- Law & Order franchise and Homicide: Life on the Street
- The character John Munch has been a mainstay on all seasons both of Homicide and Law & Order: Special Victims Unit
- The character Mike Logan (portrayed by Chris Noth) was a regular on the original series, appeared in the TV movie Exiled: A Law & Order Movie, and on a later series in the franchise, Law & Order: Criminal Intent.
- Good Luck Charlie and Shake It Up
- Drake & Josh, Zoey 101, iCarly, Victorious
- Perfect Strangers, Family Matters, Full House, Step by Step and Hangin' with Mr. Cooper.
- The X-Files, Millennium and The Lone Gunmen
- Doctor Who, K-9 and Company, Torchwood and The Sarah Jane Adventures
- That's So Raven, The Suite Life of Zack & Cody, Hannah Montana, Cory in the House, Wizards of Waverly Place, The Suite Life on Deck, I'm in the Band, Jessie and Austin & Ally
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