Paracosm

A Paracosm is a detailed imaginary world created inside one's mind. This fantasy world may involve humans, animals, and things that exist in reality; or it may also contain entities that are entirely imaginary, alien, and otherworldly. Commonly having its own geography, history, and language, the experience of such a paracosm is often developed during childhood and continues over a long period of time: months or even years.

Paracosms need not have ever been published or even put down on paper or other media -- but such cases are hard to document and share. Many are only known because they inspired or were transitioned entirely by their creator into the setting for literary or other works. A few are known only second-hand, such as Hartley Coleridge's Ejuxria or the kingdom of Borovnia created by teen murderers Juliet Hulme and Pauline Parker.

This trope is very similar to Constructed World, with which it strongly overlaps; the primary difference seems to be that Paracosms are, or at least start out as, personal and private mental constructions, created for one's own exploration, entertainment and/or satisfaction rather than with the explicit goal of telling a story set therein. A good rule of thumb, though not a perfect one, might be that a story often needs a Constructed World to be in, but while a Paracosm might become the setting for a story, it isn't necessary or guaranteed.

See also Worldbuilding.

If a Paracosm became the basis for a work, list it under the medium that the work came out in (or first came out in, if it became a multi-media franchise). If it is only known second-hand, never having become the basis of a work in its own right, list it under Real Life.

Literature

 * K.C. Remington has written over twenty books in the Webbster and Button Children's Stories series, set in a paracosm called the Big Green Woods.
 * Middle Earth, the highly detailed fantasy world created by J. R. R. Tolkien, as expressed in his novels The Lord of the Rings and The Hobbit, as well as a sizable body of writings published posthumously containing fictional histories, languages and other reference material. Tolkien had been inventing languages since his teen years, only later imagining the people who spoke them or their environment.
 * Gondal, Angria, and Gaaldine, the fantasy kingdoms created and written about in childhood by Emily, Anne, and Charlotte Brontë, and their brother Branwell, and maintained well into adulthood.
 * Austin Tappan Wright's Islandia began as a childhood paracosm.
 * The modern fantasy author Steph Swainston's world of the Fourlands is another example of an early childhood paracosm.
 * Henry Darger began writing about the Realms of the Unreal in his late teens and continued to write and illustrate it for decades. Sadly it wasn't discovered after his death, but his body of work is now celebrated as one of the greatest examples of Outsider Art.
 * As children, novelist C. S. Lewis and his brother Warren together created a paracosm called Boxen which was in turn a combination of their respective private paracosms Animal-Land and India. Lewis later drew upon Animal-Land to create the fantasy land of Narnia, which he wrote about in The Chronicles of Narnia.
 * Joanne Greenberg created a paracosm called Iria as a young girl, and described it to Frieda Fromm-Reichmann while hospitalized at Chestnut Lodge. Fromm-Reichmann wrote about it in an article for the American Journal of Psychiatry; Greenberg wrote about it as the Kingdom of Yr in her novel I Never Promised You a Rose Garden.
 * A completely fictional example would be Terabithia, a fantasy kingdom created by Jesse and Leslie in the novel (and later movie) Bridge to Terabithia.

Tabletop Games

 * Linguist and writer Professor M.A.R. Barker began developing the fantasy world Tékumel at age ten. As an adult he created the roleplaying game Empire of the Petal Throne for Tékumel, and starting in the 1980s wrote five novels set in it.

Real Life

 * English poet, biographer and essayist Hartley Coleridge initially created the land of Ejuxria in his childhood, and his brother Derwent (with whom he shared it) believed that he continued developing and maintaining it into his adulthood, but all information about it is second-hand; Coleridge never put it to paper or shared it with anyone else.
 * Borovnia, the fantasy kingdom created by Juliet Hulme and Pauline Parker in their mid-teens, as portrayed in the film Heavenly Creatures.