The history and geography of a fictional universe are well defined, and maps and timelines are often included in works set within them. Even new 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 elements of magic that don’t exist in the real world, these must adhere to a set of rules established by the author. […]
A fictional universe may even concern itself with more than one interconnected universe through fictional devices such as dreams, “time travel” or “parallel worlds”. Such a series of interconnected universes is often called a multiverse. Such multiverses have been featured prominently in science fiction since at least the mid-20th century.
Source: Fictional universe – Wikipedia