In alchemy, among the classical elements, quintessence (meaning "fifth element", along with earth, air, fire, and water) was another term for aether; it is the substance of which the heavenly bodies were supposed to be composed.
More loosely, quintessence is the purest form of a particular substance. This is seen in the Hamlet soliloquy: "The beauty of the world, the paragon of animals and yet to me what is this quintessence of dust?"