The Ethereal Plane is a misty, fog-bound dimension. It overlaps with the Material Plane and the Inner Planes, so that every location on those planes has a corresponding location on the Ethereal Plane. It is a place of echoes and fog, where hearing feels like listening to memories of sound, and sight is like looking through frosted glass.
It has also been called a plane of protomatter - raw potential before it is filtered through the Material Plane.