A fictional universe is a cohesive fictional world that serves as the setting or backdrop for one or (more commonly) multiple works of fiction. Fictional universes are most common in, but not exclusive to, the science fiction and fantasy genres. Many universes written in one or both of these genres feature physical and metaphysical laws different from our own that allow for magical, psychic and various other types of paranormal phenomena. Although these laws may not be completely internally consistent, they do allow the author to provide some textual explanation for how their imagined world differs from our own.
A fictional universe is a type of conworld (constructed world) unique to serialized, series-based, open-ended or round robin-style fiction. A fictional universe may also be called a fictional realm, imaginary realm, fictional world, imaginary world or imaginary universe. Most fictional universes are based directly or indirectly on our own universe. A fictional universe is usually differentiated from the setting of, and the cosmology established by, ancient or modern legends, myths and religions, although there are countless fictional universes that draw upon such sources for inspiration.
It can be argued that every work of fiction generates a world of its own; Robert A. Heinlein coined the neologism ficton to refer to such a world. A fictional universe is then a ficton that has an existence that goes beyond a single story, and becomes the basis either of other stories, or of games or other creations. It generally consists of a time and place that invoke a sense of a distinct world, one which is unique to the content and context of the tales that it is used to tell. Despite the name, a fictional universe does not necessarily concern an entire universe; for example, most of the television series Buffy the Vampire Slayer is set in a single Californian city, and most of the action in the Harry Potter series occurs in and around a single school. On the other hand, a fictional universe may concern itself with more than one interconnected universe; a series of interconnected universes is called a multiverse. The His Dark Materials series takes place in a fictional multiverse.
Fictional universes are sometimes shared by multiple authors, with each author's works in that universe being granted approximately equal canonical status. 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.
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. The 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 are authorized.
See Category:Fictional universes for a list of fictional universes by name and list of fictional universes for a list of fictional universes by genre.