paracosm
A detailed imaginary world, especially one created by a child.
Imaginary playmates and paracosms may chiefly represent the vast creative potential of inherently talented people, but in less elaborated forms, they may also represent what childhood imagination can offer to the growing person. Humanity has already benefited from the paracosmic visions of Plato, Thomas More, Aldous Huxley, Thomas Hobbes, Arthur Tappan Wright^([sic – meaning Austin Tappan Wright]) (Islandia), and J. R. R. Tolkien.
Where it comes from
From English para- (prefix meaning ‘alongside, beside’) (from Ancient Greek παρα- (para-, prefix meaning ‘beside’)) + English cosm(os) (“universe”) (from Ancient Greek κόσμος (kósmos, “the earth, world; universe”), ultimately from Proto-Indo-European *ḱems-, *ḱens- (“to put in order”)).
Adapted from the Wiktionary entry paracosm (CC BY-SA 4.0).