A paracosm is a detailed imaginary world. Paracosms are thought generally to originate in childhood and to have one or numerous creators. The creator of a paracosm has a complex and deeply felt relationship with this subjective universe, which may incorporate real-world or imaginary characters and conventions. Commonly having its own geography, history, and language, it is an experience that is often developed during childhood and continues over a long period of time, months or even years, as a sophisticated reality that can last into adulthood.
Source: Paracosm – Wikipedia