From Kook Science

Etheria (also referred to as the Etheric Plane, Ether World, &c.) is, according to proponents, a dimensional plane of a density distinct from our own, in which and out of which comes our dimension and others (being interpenetrating), and from whence come Etherians, including certain spirits of the departed from this dimension, as well as other interdimensional beings who travel aboard flying saucers. The exact nature and form of this dimensional plane is of varied description: for example, John Ballou Newbrough's Oahspe calls it "the midway between the planets, the highest of all heavens"; and Andrews's Universology describes the Ether World as "the Matrix or Continent of the Radiating Odylic Forces, which penetratingly and diffusively permeate it in all directions"; while Layne suggests our physical dimension is a "rarefaction," or "a net with enormous meshes," and that Etheria exists as a dimension of higher density in the "spaces" within our less dense plane.

This concept, depending on the source, may be used as an equivalent to the astral plane; when it is not used as such, it will typically be on the basis of the astral being a virtual state of our physical dimension, rather than a distinct, parallel dimension in its own right.