Pantheistic solipsism is a technical term (properly "Pantheistic multiple-ego solipsism") that has been advanced for the World as Myth idea proposed by science fiction writer Robert A. Heinlein in several of his books and stories, although the concept has little in common with either pantheism (the universe is God) or solipsism (nothing exists but my mind). Heinlein coined the phrase for the last chapter of The Number of the Beast as part of the name of a convention attended by numerous characters from different fictional universes.
The World as Myth involves the idea that a powerful author, such as Edgar Rice Burroughs, Isaac Asimov, or Heinlein himself, creates a parallel universe simply by writing about it. It incorporates the portrayal of all myths and fictional universes existing as parallel universes to our own and that persons and beings from these various “worlds” interact with one another.
For instance, in his last novels, Heinlein’s characters actually travel to and interact with the Land of Oz. Even our own world is considered an alternate (coded as "One Small Step" for the first words spoken on the moon by Neil Armstrong).
Pantheistic Solipsism is the basis for the plot of the Woody Allen film The Purple Rose of Cairo in which a fictional character in a movie comes off the screen and into the "real world". The Arnold Schwarzenegger film The Last Action Hero later re-used this concept, in an action film rather than a tragicomic love story. In a similar example, the movie Pleasantville features a pair of teenagers who stumble into a reality based on a popular 1950's television show. Neil Gaiman frequently uses pantheistic solipsism in his writing.
The World of Myth concept was used as a central theme in the book Myst: The Book of Atrus by Rand and Robyn Miller. The writers of D'ni long had the ability to forge "links" with other worlds by writing descriptions of them. These links were bridges between the writer's universe and the potential universe, having been brought into existence by being observed by the writer, as per quantum mechanics. However, the last of the writers, Ghen, believed that he was creating those universes through his writing, and thus thought he could control them by changing them subtly after forging a link. When his plans go awry, he attempts to "erase" some of his previous mistakes, and the world on the other side of the link was not only free of his mistakes, but none of the people living there had any knowledge of his existence, giving the impression that he had not erased his mistakes, but rather forged a link to an entirely different universe where the mistakes had never been made.