There are two related science definitions for the term "aether", as well as other non-scientific meanings.
(1) Aether was the fifth element in alchemical chemistry and early physics. It was the name given to the material that was believed to fill the universe beyond the terrestrial sphere. The belief in aether as an element was held by medieval alchemists, Greeks, Buddhists, Hindus, the Japanese, and the Tibetan Bon. Ancient Babylonians believed the fifth element to be the sky. The fifth element in the Chinese Wu-Xing was metal rather than aether.
(2) Aether was also considered the medium that carried light waves in space by 18th and 19th Century scientists. Luminiferous ether was proposed in order to explain the capacity of light to propagate through apparently empty space. The Michelson-Morley experiment (MMX) led scientists to realize there was no aether and that light was self-propagating.
Michelson-Morley Experiment and Aether
The MMX experiment was performed at what is now Case Western Reserve University in Cleveland, Ohio in 1887 by Albert A. Michelson and Edward Morley. The experiment used an interferometer to compare the speed of light in perpendicular directions. The point of the experiment was to determine the relative motion of matter through the aether wind or luminiferous aether. It was believed light required a medium in order to move, similar to the way sound waves require a medium (e.g., water or air) to propagate. Since it was known light could travel in a vacuum, it was believed the vacuum must be filled with a substance called aether. Since the Earth would revolve around the Sun through the aether, there would be a relative motion between the Earth and the aether (the aether wind). Thus, the speed of light would be affected by whether the light was moving in the direction of the Earth's orbit or perpendicular to it. The negative results were published in the same year and followed up with experiments of increased sensitivity. The MMX experiment led to the development of the theory of special relativity, which does not rely on any aether for the propagation of electromagnetic radiation. The Michelson-Morley experiment is considered to be the most famous "failed experiment".
(3) The word aether or ether may be used to describe apparently empty space. In Homeric Greek, the word aether refers to the clear sky or pure air. It was believed to be the pure essence breathed by gods, while man required air to breathe. In the modern usage, aether simply refers to invisible space (e.g., I lost my email to the aether.)
Alternate Spellings: Æther, ether, luminous aether, luminiferous aether, aether wind, light-bearing ether
Commonly Confused With: Aether is not the same thing as the chemical substance, ether, which is the name given to a class of compounds containing an ether group. An ether group consists of an oxygen atom connected to two aryl groups or alkyl groups.
Aether Symbol in Alchemy
Unlike many alchemical "elements", aether does not have a commonly accepted symbol. Most often, it was represented by a simple circle.