paracosm

/ˈpɛrəˌkɑzəm/
noun
  1. A detailed imaginary world created in a person's mind, often developed during childhood.
    • As a child, she invented a paracosm with its own language and geography.
    • The siblings spent hours adding new characters to their shared paracosm.
    • Many authors use a paracosm as the setting for their fantasy novels.
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