The Great Wheel is a term from Dungeons & Dragons that came into being with the AD&D setting Planescape. It refers to the structure of the multiverse, as D&D ultimately arranged its cosmology in a pattern resembling a set of consecutive rings or circles; the Prime Material Plane at the center, bordered by the Ethereal Plane, and then by the Elemental Planes, which are bordered in turn by the Astral Plane - these together make up the "Inner Planes". After that comes the assorted alignment-based "Outer" Planes, divided into the Good-based Upper Planes, the Neutral-based Middle Planes, and the Evil-based Lower Planes.
In 4th edition, the Great Wheel was replaced by the World Axis cosmology, and even though instructions on how to reshape the latter into the former were provided, it was the source of much uproar.
In 5th edition, the Great Wheel returned, but with some significant changes based on the World Axis cosmology:
- The Feywild and the Shadowfell coterminous planes were added to the planes as a whole, residing as analogues to the Prime Material Plane.
- The Elemental Planes were reshaped so that they were physically interlinked, with the former paraelemental planes literally being the regions where two elements intersected. Additionally, the layout of the Elemental Planes were reshaped to be more like Creation in Exalted; close to the Prime, the Elemental Planes resemble the Prime, but with a specific elemental trait dominant, but go deeper and the other elements fall away until you have nothing but a pure expanse of elemental matter, akin to approaching the Elemental Poles in Exalted. Go beyond the "deep elemental", though, and you break into the Elemental Chaos, the writhing, Limbo-esque plane where all elemental matter originates from and which serves a border between the Inner and Outer Planes.
- The Positive and Negative Energy Planes were removed from the Elemental Planes array (which removed the Quasielemental Planes from existence) and instead made the extreme "north" and "south" of the planar map, beyond the Outer and Inner Planes alike.
Pathfinder doesn't use the Great Wheel, instead using the inspired-by but different Great Beyond, which has a lot of equivalents but ditches the balance angle and in some cases have significantly different equivalents (the Outlands are replaced by the Boneyard and River of Souls) and relations to each other (the Maelstron, Limbo's equivalent, is by far the largest plane, rivaled only by the Abyss, and created most of the others out of it's substance. Axis, the Mechanus equivalent, is an expansionist city founded by lawful planars from outside the known universe who made Inevitables to build it outward. Etc.).
The Great Wheel is intimately tied up with the Blood War.