What is "planescape"

Planescape is a campaign setting for the Dungeons & Dragons fantasy role-playing game, originally designed by Zeb Cook. The Planescape setting was published in 1994. As its name suggests, the setting crosses and comprises the numerous planes of existence, encompassing an entire cosmology called the Great Wheel, as originally developed in the Manual of the Planes by Jeff Grubb. This includes many of the other Dungeons & Dragons worlds, linking them via inter-dimensional magical portals.