Ether is a word derived from the Greek aithēr (αἰθήρ), meaning "pure, fresh air" or "clear sky", and refers to a postulated space-filling substance or field. Aristotle included aether among the four terrestrial elements, thus making of it the fifth one (the quintessence). In the late 19th century the "luminiferous ether" was a light-bearing element postulated by scientists to describe a medium for the propagation of light, theory that was later discarded.
The Theosophical literature also regards ether as the fifth element (out of seven) although only four are at this point completely manifested:
Seven Elements on our Earth. Of these, four elements are now fully manifested, while the fifth —Ether— is only partially so, as we are hardly in the second half of the Fourth Round, and consequently the fifth Element will manifest fully only in the Fifth Round.
The ether of the 19th century science is described by Helena Petrovna Blavatsky as a kind of "material" ether:
Ether. Students are but too apt to confuse this with Akasa and with Astral Light. It is neither, in the sense in which ether is described by physical Science. Ether is a material agent, though hitherto undetected by any physical apparatus; whereas Akasa is a distinctly spiritual agent, identical, in one sense, with the Anima Mundi, while the Astral Light is only the seventh and highest principle of the terrestrial atmosphere, as undetectable as Akasa and real Ether, because it is something quite on another plane.
Thus, the term may also refer to the "real ether", this being the more spiritual principle called the Astral Light:
Ether, this hypothetical Proteus, one of the “representative Fictions” of modern Science—which, nevertheless, was so long accepted—is one of the lower “principles of what we call PRIMORDIAL SUBSTANCE (Akâsa, in Sanskrit), one of the dreams of old, and which has now become again the dream of modern science. It is the greatest, just as it is the boldest, of the surviving speculations of ancient philosophers. For the Occultists, however, both ETHER and the Primordial Substance are a reality. To put it plainly, ETHER is the Astral Light, and the Primordial Substance is AKÂSA, the Upadhi of DIVINE THOUGHT.
- Helena Petrovna Blavatsky, The Secret Doctrine vol. I, (Wheaton, IL: Theosophical Publishing House, 1993), 140.
- Helena Petrovna Blavatsky, Collected Writings vol. IV (Wheaton, IL: Theosophical Publishing House, 1981), 124.
- Helena Petrovna Blavatsky, The Secret Doctrine vol. I, (Wheaton, IL: Theosophical Publishing House, 1993), 326.