Short answers, pulled from the story.
In ancient Greek philosophy, aether was the material believed to fill the region of the universe beyond the terrestrial sphere. Aristotle introduced it as a fifth element in his work On the Heavens, distinct from earth, water, fire, and air. It was neither hot nor cold, neither wet nor dry, and moved only in perfect circles.
Quintessence is the Latinate name for aether used by medieval alchemists. The word aether derives from ancient Greek and was applied by Aristotle to the substance of the heavens; quintessence, from the Latin for fifth essence, was adopted in medieval Europe and associated with alchemical practice, including medicinal uses. Both names refer to the same underlying concept of a fifth element.
The Michelson-Morley experiment conclusively found no evidence for the motion of aether. Its results influenced many physicists of the time and contributed to the development of Einstein's theory of special relativity, which showed that Maxwell's equations do not require aether for the transmission of electromagnetic or gravitational forces.
Newton described aether as a medium that flowed continuously downward toward the earth's surface and was partially absorbed and partially diffused. He associated this circulation with the force of gravity. He explained this model in his letter to Robert Boyle in 1679 and incorporated it into Philosophiæ Naturalis Principia Mathematica, published in 1687.
Aether was a deity in traditional Greek mythology, the son of Erebus and Nyx. He personified the pure, bright upper air breathed by the gods, in contrast to the ordinary air inhaled by mortals.
A proposed model of dark energy has been named quintessence by its proponents, in honor of the classical fifth element. It relates to a hypothetical form of dark energy postulated to explain observations of an accelerating universe, and has also been called a fifth fundamental force.