αἰθήρ

According to ancient and medieval science, aether (Greek: αἰθήρ aithēr[1]), also spelled æther or ether, also called quintessence, is the material that fills the region of the universe above the terrestrial sphere.[citation needed] The concept of aether was used in several theories to explain several natural phenomena, such as the traveling of light and gravity. In the late 19th century, physicists postulated that aether permeated all throughout space, providing a medium through which light could travel in a vacuum, but evidence for the presence of such a medium was not found in the Michelson–Morley experiment.[2]

 - from Wikipedia