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Worldbuilding is the process of creating a complete imaginary setting for a story, novel, film, game, etc., particularly in the science fiction, fantasy, or horror genres. Probably the longest, most ambitious worldbuilding ever undertaken was when Tolkien created the languages and mythology of Middle-earth. However, he would not have used the word; the earliest cite listed in Brave New Words is 1965.
The term is used by both professional writers and fanfic writers. In fanfic, the term is especially applicable to writing AUs, but canon-compliant stories may also involve worldbuilding if the writer examines gaps in canon or sets their story in the future.