It is difficult to determine what constitutes a fictional universe, but whether it is contained in a single work, or consists of a succession of works — as frequently happens in fantasy or science fiction series — the universe is self-consistent and follows an established set of rules. Its history and geography are well-defined, and even languages may be constructed. When subsequent works are written within the same universe, care must be taken to ensure that established rules of the canon are not violated.
Sir Thomas More's Utopia is one of the earliest examples of a cohesive imaginary world with its own rules and functional concepts, but it comprises only one small island. Later fictional universes, like Robert E. Howard's Conan the Cimmerian stories, are global in scope, and some, like Star Wars, Honorverse, or the Lensman series, are galactic or even intergalactic. A fictional universe may even concern itself with more than one interconnected universe through theoretically viable devices such as "parallel worlds" or universes, and a series of interconnected universes is called a multiverse. Such multiverses have been featured prominently in science fiction since at least the mid-20th century, notably in the classic Star Trek episode, "Mirror, Mirror", which introduced the mirror universe in which the crew of the Starship Enterprise were brutal, rather than civilized, and in the mid-1980s comic book series, Crisis on Infinite Earths, in which countless parallel universes were destroyed. The Hitchhiker's Guide to the Galaxy, when considered as all 5 books together, flits back and forth between different universes, or perhaps it is more accurate to say, flits through different timelines and different dimensions involving different states of existence for the characters and for the earth itself.
In most small-scale fictional universes, general properties and timeline events fit into a consistently organized continuity. However, in the case of universes or universes that are rewritten or revised by different writers, editors or producers, this continuity may be violated, by accident or by design—film productions are notorious for altering fictional canon of written series.
The occasional publishing use of retroactive continuity (retcon) often occurs due to this kind of revision or oversight. Members of fandom often create a kind of fanmade canon (fanon) to patch up such errors; "fanon" that becomes generally accepted sometimes becomes actual canon. Other fanmade additions to a universe (fan fiction, pastiche, parody) are usually not considered canonical unless they get authorized.
Other universes are created by one or several authors but are intended to be used non-canonically by others, such as the fictional settings for games, particularly role-playing games and video games. Settings for the role-playing game Dungeons & Dragons are called campaign settings; other games have also incorporated this term on occasion. Virtual worlds are fictional worlds in which online computer games, notably MMORPGs and MUDs, take place. A fictional crossover occurs when two or more fictional characters, series or universes cross over with one another, usually in the context of a character created by one author or owned by one company meeting a character created or owned by another. In the case where two fictional universes covering entire actual universes cross over, physical travel from one universe to another may actually occur in the course of the story. Such crossovers are usually, but not always, considered non-canonical by their creators or by those in charge of the properties involved.
An example of this from television is the "Tommy Westphall universe", when the final episode of the medical drama St. Elsewhere revealed that the entire series had in fact taken place within the mind of an autistic child named Tommy Westphall. This in turn meant that the series Homicide: Life on the Street, which featured many shared characters with St Elsewhere, must also have taken place within his mind, as had the series Law & Order and its subsequent spinoffs, since they had crossed over with Homicide. This "universe" in fact has been extended to hundreds of other interconnected shows, as diverse as Newhart and Star Trek, to the point that, according to series creator Tom Fontana, "something like 90 percent of all television took place in Tommy Westphall's mind. Other, less extreme examples of this include the drama ER, which takes place in the same fictional universe as the TV series Third Watch, and the sitcoms Friends, Seinfeld and Mad About You, which all take place within the same fictionalised New York City