Shared universe

A shared universe or shared world is a fictional universe from 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. It is common in genres like science fiction. It differs from collaborative writing in which multiple artists are working together on the same work and from crossovers where the works and characters are independent except for a single meeting.
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. A specific kind of shared universe that is published across a variety of media (such as novels and films), each of them contributing to the growth, history, and status of the setting is called an "imaginary entertainment environment."
The term has also been used in a wider, non-literary sense to convey interdisciplinary or social commonality, often in the context of a "shared universe of discourse".
Definitions.
Comics historian Don Markstein first defined the term in a 1970 article in "CAPA-alpha".
Markstein's criteria.
Fiction in some media, such as most television programs and many comic book titles, is understood by viewers or readers 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, often being one-offs.
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 print publications.
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 prevented 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/based upon comics.
Within comics, the term "shared universe" has been used 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.
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 Comics 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.
Universes in films and television.
Universes in films (or television) mostly consist of a franchise featuring an umbrella of multiple franchises (film or television) set within the same continuity, each franchise within telling its own stand-alone story focusing on a different character (or group of characters), as well as featuring its own cast, directors, and writers, while also being a part of a coherent, non-contradictory continuity shared with the other works. Fictional universes with major presence in films are referred to as cinematic universes, while fictional universes with major presence in television are referred to as television universes. Universes with major presence in films and television, generally are referred as cinematic universes as that's the medium that reaches the most audiences. Some film and television universes are accompanied by video games, and print works such as novels or comics, telling additional canonical stories set within the same continuity.
Universes in films.
An early universe in cinema history was the "Universal Monsters" series of "Dracula", "Frankenstein", "The Invisible Man" and "The Wolf Man", which ran from 1931 to 1951 and featured recurring cast and characters.
Some universes in film have originated as film adaptations of novels, such as the "James Bond" film series based on the spy novels by Ian Fleming. The films adapt some elements from the novels, however they are not direct adaptations, additionally they were not adapted in the same order as the books released. The first film in the series is "Dr. No" (1962), and has produced 23 sequels, with the most recent one being "Spectre" (2015). Due to the series spanning more than 50 years, James Bond has been portrayed by six actors, all the other frequent cast of characters has also gone through replacements, additionally after all Fleming's novels were adapted the series shifted towards original scripts. However, all films are set within a single, coherent fictional continuity, even if the writers, directors and cast change between films. Aspects of the fictional universe are retained between films.
The "Planet of the Apes" film series also originated as a film adaptation of the novel of the same name, taking many creative liberties. The original film titled "Planet of the Apes" (1968) focused on present-day astronaut George Taylor landing on a mysterious planet ruled by apes, which is revealed to be a future planet Earth at the end of the film. The sequel "Beneath" focused on Brent, an astronaut sent on a rescue mission to save Taylor. The third film "Escape" introduced time travel and shifted the focus towards the apes Zira and Cornelius, who appeared as supporting characters in the previous films, as they travel to the past in Taylor's spaceship. The fourth and fifth films, "Conquest" and "Battle", focus on Zira and Cornelius' son Cesar leading the uprising against the humans and to the future depicted in the original film. A television series and an animated series are also considered part of the story. The origin of the "Planet of the Apes" in the original timeline before the time travel occurred was explained in a prequel-reboot film series with the ape Cesar becoming the main protagonist again, with the story focused on his life from childhood to an old ape, and how the conflict between the apes and the humans started.
The "Star Wars" film series was created by George Lucas, produced by his self-funded production company Lucasfilm. It originated with the film "Star Wars" (1977), followed by two sequels "The Empire Strikes Back" (1980) and "Return of the Jedi" (1983); together, those three films are known as the original trilogy and focuses on Luke Skywalker, Han Solo and Princess Leia. The prequel trilogy composed of ' (1999), ' (2002), and "" (2005) expanded the series into a Saga, and focused on Luke Skywalker's father Anakin Skywalker, new cast members portrayed younger versions of characters from the original trilogy, while other actors returned to their original trilogy roles. 
On television, the lore was expanded through animation, the animated film titled ' (2008) served as the pilot of an (2008–2014, 2020). Lucas was deeply creatively involved in the previously mentioned works, but he ceased creative involvement with the "Star Wars" franchise in 2014. Lucasfilm announced that from April 2014, only such previously mentioned works would be considered canonical, along with all of the fictional works released after such date. The animated series "Star Wars Rebels" (2014–2018), was the first work released after. A sequel trilogy formed by ' (2015), ' (2017) and ' (2019) featured many returning cast members along with newcomers. The films’ main saga is conformed of the original, prequel, and sequel trilogies. A concurrent spin-off film series, known as the anthology films, expands the stories of plot points and characters from the main series. 
Additionally the Star Wars series expanded into video games, comics, and novels, telling original stories based on the franchise, classifying it as an imaginary entertainment environment, where the films share the same continuity as all the other media formats, regardless of the different media formats. Lucasfilm's "Story Group" coordinates a cohesive story-telling and ensures consistency and synergy by avoiding plot holes between all the film and non-film works. The "Star Wars Legends" brand is used to brand all the re-prints of non-canonical works (television films, animated series, video-games, comics, and novels) of the franchise which were produced and/or ceased production before April 2014. Lucas himself considered the "Legends" works as non-canonical because he considered them to diminish his films meaning by not adhering to his vision. Albeit, Lucas did include a few of the "Legends" elements that he supported within his works, such as characters and places which made those included elements canonical. Storytellers after Lucas have also incorporated "Legends" elements in their stories, sometimes with the help of the original creators.
As for comic book-based films, there are two cinematic universes based on Marvel Comics characters, both set within a different continuity. The "X-Men" film series, which originated in 2000, was the longest-running superhero film franchise to be set within the same continuity. The Marvel Cinematic Universe has the most films, as well as multiple TV shows and a separate run of tie-in comics that co-exist and share the same continuity, making the MCU an imaginary entertainment environment. The DC Extended Universe is a movie franchise that encompasses different movie series based on the DC Comics characters, all of them sharing a continuity.
Writer/director Kevin Smith created a fictional universe used in several of his films, as well as comics and a television series: the View Askewniverse, which is named for Smith's production company, View Askew Productions. The characters Jay and Silent Bob (played by Smith) appear in almost all the View Askewniverse media, and characters from one story often reappear or are referred to in others. Recurring characters, settings, and motifs of the View Askewniverse first appeared in Smith's debut film, "Clerks", in 1994.
Among the most successful novel-to-film adaptations are the film trilogies of "The Hobbit" (2012–2014) and "The Lord of the Rings" (2001–2003), all of which were directed by Peter Jackson, while serving as faithful adaptations of the novels by J. R. R. Tolkien (especially "The Lord of the Rings"). The first work in the series was "The Hobbit novel" published in 1937. The novels focused on the hobbits Bilbo Baggins and his nephew Frodo Baggins, as well as the wizard Gandalf, among other species such as dwarves and elves. Videogames taking place between the films of the saga have been produced.
A cinematic universe consisting of films whose titles are the names of songs by Simon and Garfunkel was proposed and publicly discussed by movie creators in 2017. Filmmaker Edgar Wright wrote that the Simon and Garfunkel cinematic universe could begin with his movie "Baby Driver" and Marc Webb's "The Only Living Boy in New York", and Wright suggested creation of a film named "So Long, Frank Lloyd Wright". Subsequently, other writers, directors, and actors expressed their interest in making films within the universe, such as Rian Johnson: "Keep the Customer Satisfied"; Lin-Manuel Miranda: "Cecilia"; Marc Webb and Dwayne Johnson: "I Am a Rock".
Universes in television.
This refers to universes that are based on television shows without any films. Television series may lead to a spin-off series set in the same universe, often focusing on a single character from the original. The American sitcom "Cheers" led to two spin-off series based on its characters, in the form of "Frasier" and "The Tortellis".
A significant example of shared universes among television shows is the Tommy Westphall Universe Hypothesis, which suggests that hundreds of American television series take place in the same universe. It builds from the assumption that when an actor playing a character from one series guest-stars in a second series, in character, both of those series must take place in the same universe. The theory takes its name from a character in the final episode of "St. Elsewhere", where the common interpretation of the events of that finale is that the entire "St. Elsewhere" universe – including all connected series – exist only within Westphall's imagination.
The spin-off media from "Doctor Who", known as the "Whoniverse", 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.
Television network The CW houses the Arrowverse, a shared universe of television shows based on characters from DC Comics.
Universes in animated films and animated series.
The Mickey Mouse universe dates back to the 1930s when the animated cartoon was expanded into a newspaper strip. Although the characters occasionally portray other roles and with other names, the writers address this discrepancy by thinking of the characters as being "employed" by Disney as actors. Walt Disney, when asked whether or not Mickey Mouse and Minnie Mouse were married, replied that the mice were indeed married in their "private li[ves]", but that they sometimes appeared as boyfriend and girlfriend for "screen purposes." The Mickey Mouse universe also includes the Donald Duck universe as a subset.
The Pixar universe is an elaborate fan theory suggesting that all Pixar animated movies take place in the same universe. At the 2015 D23 Expo, during the "Pixar Secrets Revealed" panel, director Mark Andrews rejected the theory, with "Inside Out" co-director Ronnie del Carmen adding "Do you know what kinds of meetings we'd have to have to make sure all our movies line up?!"
Originating in video games and 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 with some limited publication. Other early examples include the Dargon Project and Devilbunnies.
Other media.
The 2000 musical "Seussical" presented several works of Dr. Seuss as taking place in the same fictional world.
Hasbro toy products including G.I. Joe and Transformers are considered by their manufacturer to exist fictionally within the Hasbro Universe.