Ethereal Plane
The Ethereal Plane is one of the Inner Planes in the Great Wheel cosmology. In fact, it is the second-innermost of the Inner Planes, being the Plane closest to the Prime Material. Described as a grey and misty realm that exists just slightly out of phase with the physical universe, the Ethereal Plane is mostly ignored, save for the fact it's the "default plane" for ghosts, and that a skilled wizard or other caster can traditionally use the Ethereal Plane as a way to get around physical obstacles.