paracosm
According to Wikipedia:
A paracosm is a detailed imaginary world thought generally to originate in childhood. Their conception is spontaneous, and 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. This results in a very intimate but also often secret relationship with this world, meaning that few people ever know of its existence or contents. Commonly having its own geography, history, and language, it is an experience that is commonly developed during childhood, but becomes a rarer development with age. It continues over many months or even years, rarely lasting into adulthood. A paracosm can lead to the development of a heterocosm, which is a polished and edited version of a paracosm intended for sharing and publishing, such as some works of the Brontës.