fictional universe

It is difficult to determine what actually constitutes a "fictional universe." 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. Some, like Robert E. Howard's Conan the Cimmerian stories, are global in scope, and some, like Star Trek and Star Wars 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.
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