Fictional universe
A fictional universe is the internally consistent fictional setting used in a narrative work or work of art, most commonly associated with works of fantasy and science fiction. Fictional universes appear in novels, comics, films, television shows, video games, art, and other creative works.A fictional universe may be an alternative version of the real world, differing only in the particulars of the story. All fiction, in this sense, is set in a fictional universe, since at least some of its characters, events, and places are not real; the term "fictional universe", however, is usually not applied to worlds that do not contain speculative elements. Thus a world that is presented as the same as our own world but which has superheroes or magic is a fictional universe. Such "alternate reality" settings are common for comic books (as in Superman), low fantasy (as in Harry Potter), and near-future science fiction (as in Star Trek).
When the setting of a fictional universe is not presented as our own world but as its own distinct world, it is often instead called a fictional world or "fantasy world". In science fiction such a fictional world may be a remote alien planet or galaxy with little apparent relationship to the real world (as in Star Wars); in fantasy it may be a greatly fictionalized or invented version of earth's distant past or future (as in The Lord of the Rings). When such a world is meant to have no connection to our own world (in effect, our world does not exist in that world's reality) or is presented as a reality that can only be accessed from our own by a portal, it is sometimes called a secondary world; such settings are common in high fantasy (as in The Chronicles of Narnia, Earthsea, and Discworld). A fictional world that is meant to exist inside the real world (as in the Land of Oz or the Neverland) may be termed a fictional realm.
When a franchise of related works has two or more alternative fictional universes that are each internally consistent but which are not fully consistent with one another (as by having distinct plotlines and characters, for example between a comic and its film adaptation), each such alternative universe may be referred to as a (fictional) continuity.

Universe vs. setting
A famous example of a detailed fictional universe is Arda (more popularly known as Middle-earth), of J. R. R. Tolkien's books The Lord of the Rings, The Hobbit, and The Silmarillion. He created first its languages and then the world itself, which he states was "primarily linguistic in inspiration and was begun in order to provide the necessary 'history' for the Elvish tongues."A modern example of a fictional universe is that of the Avatar film series, as James Cameron invented an entire ecosystem, with a team of scientists to test whether it was viable. Additionally, he commissioned a linguistics expert to invent the Na'vi language.Virtually every successful fictional TV series or comic book develops its own "universe" to keep track of the various episodes or issues. Writers for that series must follow its story bible.

Fictional continuity
In a 1970 article in CAPA-alpha, comics historian Don Markstein provided a definition of fictional universe meant to clarify the concept of fictional continuities. According to the criteria he imagined:

If characters A and B have met, then they are in the same universe; if characters B and C have met, then, transitively, A and C are in the same universe.
Characters cannot be connected by real people—otherwise, it could be argued that Superman and the Fantastic Four were in the same universe, as Superman met John F. Kennedy, Kennedy met Neil Armstrong, and Armstrong met the Fantastic Four.
Characters cannot be connected by characters "that do not originate with the publisher"—otherwise it could be argued that Superman and the Fantastic Four were in the same universe, as both met Hercules.
Specific fictionalized versions of real people—for instance, the version of Jerry Lewis from DC Comics' The Adventures of Jerry Lewis, who was distinct from the real Jerry Lewis in that he had a housekeeper with magical powers—can be used as connections; this also applies to specific versions of public-domain fictional characters, such as Marvel Comics' version of Hercules or DC Comics' version of Robin Hood.
Characters are only considered to have met if they appeared together in a story; therefore, characters who simply appeared on the same front cover are not necessarily in the same universe.

Collaboration
Fictional universes are sometimes shared by multiple prose authors, with each author's works in that universe being granted approximately equal canonical status. For example, Larry Niven's fictional universe Known Space has an approximately 135-year period in which Niven allows other authors to write stories about the Man-Kzin Wars. Other fictional universes, like the Ring of Fire series, actively court canonical stimulus from fans, but gate and control the changes through a formalized process and the final say of the editor and universe creator.

See also
References

Alberto Manguel and Gianni Guadalupi: The Dictionary of Imaginary Places, New York : Harcourt Brace, c2000. ISBN 0-15-100541-9
Brian Stableford: The Dictionary of Science Fiction Places, New York : Wonderland Press, c1999. ISBN 0-684-84958-5
Diana Wynne Jones: The Tough Guide to Fantasyland, New York : Firebird, 2006. ISBN 0-14-240722-4, Explains and parodies the common features of a standard fantasy world
George Ochoa and Jeffery Osier: Writer's Guide to Creating A Science Fiction Universe, Cincinnati, Ohio : Writer's Digest Books, 1993. ISBN 0-89879-536-2
Michael Page and Robert Ingpen : Encyclopedia of Things That Never Were: Creatures, Places, and People, 1987. ISBN 0-14-010008-3