Imaginary universe
An imaginary universe is a more or less complete fictional realm such as Middle-earth in the works of J. R. R. Tolkien; in the Darkover novels and short stories of Marion Zimmer Bradley; or in the Earthsea Trilogy of Ursula K. Le Guin.A well executed imaginary world features internally consistent physical and psychic laws different from our own allowing often for magical and psychic phenomena.
The television show Buffy the Vampire Slayer is an excellent example; as is the Star Trek universe. Star Trek is a more mundane example which while rooted in reality nevertheless in its creation of species such as the Klingon and constant ever-novel elementary particles gradually becomes an imaginary universe of the future.
Universal properties and timeline events should fit into a consistent, well organized continuity. This is not always the case, as retcons, alternate timelines and alternate dimensions exist, as well as fictional crossovers and other events which may not always be canon.
Imaginary universes in literature
Imaginary universes in film and television
Imaginary universes in comic books
Imaginary universes in role-playing games
Imaginary universes in video games