Planescape is a campaign setting
Campaign setting
A campaign setting is usually a fictional world which serves as a setting for a role-playing game or wargame campaign. A campaign is a series of individual adventures, and a campaign setting is the world in which such adventures and campaigns take place...

 for the Dungeons & Dragons
Dungeons & Dragons
Dungeons & Dragons is a fantasy role-playing game originally designed by Gary Gygax and Dave Arneson, and first published in 1974 by Tactical Studies Rules, Inc. . The game has been published by Wizards of the Coast since 1997...

Fantasy is a genre of fiction that commonly uses magic and other supernatural phenomena as a primary element of plot, theme, or setting. Many works within the genre take place in imaginary worlds where magic is common...

 role-playing game
Role-playing game
A role-playing game is a game in which players assume the roles of characters in a fictional setting. Players take responsibility for acting out these roles within a narrative, either through literal acting, or through a process of structured decision-making or character development...

, 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
Plane (Dungeons & Dragons)
The planes of the Dungeons & Dragons roleplaying game constitutes the multiverse in which the game takes place.In the earliest versions of Dungeons & Dragons, the concept of the Inner, Ethereal, Prime Material, Astral and Outer Planes was introduced; at the time there were only four Inner Planes...

, encompassing an entire cosmology
Cosmology is the discipline that deals with the nature of the Universe as a whole. Cosmologists seek to understand the origin, evolution, structure, and ultimate fate of the Universe at large, as well as the natural laws that keep it in order...

 called the Great Wheel, as originally developed in the Manual of the Planes
Manual of the Planes
The Manual of the Planes is a manual for the Dungeons and Dragons role-playing game. This text addresses the planar cosmology of the game universe....

by Jeff Grubb
Jeff Grubb
Jeff Grubb is an author and game designer. He has worked on a number of computer and role-playing games and has written a number of successful novels, short stories and comics...

. This includes many of the other Dungeons & Dragons worlds, linking them via inter-dimensional magical
Magic (paranormal)
Magic is the claimed art of manipulating aspects of reality either by supernatural means or through knowledge of occult laws unknown to science. It is in contrast to science, in that science does not accept anything not subject to either direct or indirect observation, and subject to logical...