Ether

The clear sky; the upper air, or the airwaves; outer space. A thin, subtle matter, much finer and rarer than air, which, some philosophers suppose, begins from the limits of the atmosphere and occupies the heavenly space. The element believed in ancient and medieval civilizations to fill all space above the sphere of the moon and to compose the stars and planets. An all-pervading, infinitely elastic, mass less medium formerly postulated as the medium of propagation of electromagnetic waves. The fifth and highest element after air and earth and fire and water; was believed to be the substance composing all heavenly bodies.

(Source: http://www.tu-harburg.de/rzt/rzt/it/Ether.html )