Ethereal comes from Latin aether "the upper pure, bright air; sky, firmament," from Greek aither "upper air; bright, purer air; the sky" (opposed to aer "the lower air"), from aithein "to burn, shine," from *aidh- "to burn."
Also known as the Vital body. It is the subtle aspect of the physical body, composed of the energy or vital force that provides life to the physical body. In Tibetan Buddhism, the vital body is known as the subtle body (lus phra-mo). The Ethereal / Vital Body corresponds to the fourth dimension, which in Kabbalah is called Yesod (foundation).
"It is written that the vital body or the foundation of organic life within each one of us has four ethers. The chemical ether and the ether of life are related with chemical processes and sexual reproduction. The chemical ether is a specific foundation for the organic chemical phenomena. The ether of life is the foundation of the reproductive and transformative sexual processes of the race. The two superior ethers, luminous and reflective, have more elevated functions. The luminous ether is related with the caloric, luminous, perceptive, etc., phenomena. The reflective ether serves as a medium of expression for willpower and imagination." - Samael Aun Weor, The Gnostic Bible: The Pistis Sophia Unveiled