The fifth Classical Element is known by various names: Aether (Greek αἰθήρ), Idea (Greek ίδέα), quintessence, or ίερόν, (Greek hieron "a divine thing").
According to ancient and medieval science, Aether (Greek αἰθήρ, aithēr), also spelled ether, is the material that fills the region of the universe above the terrestrial sphere. Aristotle included ίδέα, idea, as a fifth element distinct from the other four, Earth, Water, Air, and Fire.
Plato and Aristotle referred to Aether as "Idea", and in this sense, it may be regarded as that which exists outside the material world (i.e. thought processes, mathematical algorithms, etc.). Aether was also called quintessence (from quinta essentia, "fifth element"). Quintessence was also thought to be heavenly, i.e. not of the material world (matter). Quintessence was also said to have the power of life. Its Platonic solid was the dodecahedron.
Reference:
http://en.wikipedia.org/wiki/Aether_%28classical_element%29
