Plane of Existence

A Plane of Existence is a domain within the universe that spans a subset of the universe.

There is much debate as to which planes of existence, other than the Material Plane, do in fact exist.

Material Plane
The Material Plane is widely considered to exist.

Ethereal plane
The Ethereal Plane, occasionally known (more ambiguously) as the Immaterial Plane, is also almost universally recognized to exist, and 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.

The Abyss
The Abyssal plane, more commonly known as the Abyss, is a theorized plane of existence devoid of any matter.

Spiritual Plane
The Spiritual Plane is a theorized plane of existence, the residence of deities and deific beings.