Expanded Universe (or Extended Universe) is the term used for (usually) unofficial, non-canon additions to an imaginary universe.
Nearly every popular series has such additions, usually in the forms of comics or novels based within the same setting as popular movies or television shows.
This is not the same as an adaptation, which is a retelling of the same story, usually on a different medium.
The term is heavily used in Star Wars fandom. It is notable because most Star Wars fans ascribe a continuity between the Star Wars movies and the Expanded Universe, with claims that George Lucas canonized all Star Wars novels as to being the true sequels or prequels to the normal Star Wars movies. This causes Star Wars works of fiction to make far more of an attempt to be in the same continuity, and featuring the same characters, than other novelizations of science fiction. This connection is confirmed by Lucasfilm, with four levels of canon; the vast majority of Star Wars material falls on the "C-Canon" level. Many elements created in the Star Wars Expanded Universe have made it into the films, such as the planet Coruscant for instance.
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