Ether is the matter, or lack thereof, that permeates the multiverse. In its broadest sense, it also constitutes a fourth dimension or coordinate axis across which one may travel in addition to the ordinary three spatial dimensions of height, width, and depth. Most magical means of travel, including wormholes, portals, and traversing the space between planes involve piercing the ether.
The plane of Limbo is composed chiefly of ether.