Secondary world

A fictional universe is a self-consistent imaginary setting with events, and often other elements, that differ from the real world. It may also be called an imagined, constructed or fictional realm (or world). Fictional universes may appear in novels, comics, films, television shows, video games and other creative works.

A fictional universe can be almost indistinguishable from the real world, except for the presence of the invented characters and events that characterize a work of fiction; at the other extreme it can bear little or no resemblance to reality, with invented fundamental principles of space and time.

The subject is most commonly addressed in reference to fictional universes that differ markedly from reality, such as those that introduce entire fictional cities, countries, or even planets, or those that contradict commonly known facts about the world and its history, or those that feature fantasy or science fiction concepts such as magic or faster than light travel—and especially those in which the deliberate development of the setting is a substantial focus of the work.

What distinguishes a fictional universe from a simple setting is the level of detail and internal consistency. A fictional universe has an established continuity and internal logic that must be adhered to throughout the work and even across separate works. So, for instance, many books may be set in conflicting fictional versions of Victorian London, but all the stories of Sherlock Holmes are set in the same Victorian London. However, the various film series based on Sherlock Holmes follow their own separate continuities, and so do not take place in the same fictional universe.

The history and geography of a fictional universe are well-defined, and maps and timelines are often included in works set within them. Even languages may be constructed. When subsequent works are written within the same universe, care is usually taken to ensure that established facts of the canon are not violated. Even if the fictional universe involves concepts such as magic that don't exist in the real world, these must adhere to a set of rules established by the author.

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 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."

...
Wikipedia