Ethereal Plane

The Ethereal Plane, occasionally known (more ambiguously) as the Immaterial Plane, is a Plane of Existence.

Place in the cosmology
The Ethereal Plane is theorized to be superimposed over the Material plane.

The fundamental difference between the Ethereal Plane and the Material Plane is that an observer in the Ethereal Plane could perceive and interact with beings and objects in the Material Plane, and even the Material Plane in its entirety, while an observer in the Material Plane can only rarely perceive -- let alone interact with -- beings and objects in the Ethereal Plane.