According to ancient and medieval science, Ether is the material that fills the region of the universe above the terrestrial sphere. It was imagined in Greek mythology to be the pure essence where the gods lived and which they breathed, analogous to the air breathed by mortals.
Aristotle included Ether in the system of the classical elements of Ionic philosophy as the quintessence, on the principle that the four terrestrial elements were subject to change and moved naturally in straight lines while no change had been observed in the celestial regions and the heavenly bodies moved in circles.
In Aristotle’s system Ether had no qualities, was neither hot, cold, wet, or dry, and was incapable of change. By its nature it moved in circles. Medieval scholastic philosophers granted Ether changes of density in which the bodies of the planets were considered to be denser than the medium which filled the rest of the universe.
Early modern physics proposed the existence of a medium of the Ether meaning upper air or pure, fresh air, a space filling substance or field, thought to be necessary as a transmission medium. The assorted Ether theories embody the various conceptions of this medium and substance. This early modern Ether has little in common with the Ether of classical elements from which the name was borrowed.
Although hypotheses of the Ether vary somewhat in detail they all have certain characteristics in common. Essentially it is considered to be a physical medium occupying every point in space, including material bodies. A second essential feature is that its properties gives rise to the electric, magnetic and gravitational potentials and determines the propagation velocity of their effects.
Therefore the speed of light and all other propagating effects are determined by the physical properties of the Ether at the relevant location, analogous to the way that gaseous, liquid and solid media affect the propagation of sound waves.
The Ether is considered the overall reference frame for the universe and thus velocities are all absolute relative to its rest frame. Therefore, any physical consequences of those velocities are considered as having absolute or real effects.
Recent Ether theories of velocity effects, phenomenon of gravitation and planetary motion, creation of proton, of stars and planets, etc., exist but are not generally accepted by the mainstream scientific community.
John Bell, interviewed by Paul Davies in The Ghost in the Atom has suggested that an Ether theory allows a reference frame in which signals go faster than light. Bell suggests the Ether was wrongly rejected on purely philosophical grounds, in that what is unobservable does not exist.
Einstein found the non-Ether theory simpler and more elegant, but Bell suggests that doesn’t rule it out. Besides the arguments based on his interpretation of quantum mechanics, Bell also suggests resurrecting the Ether because it is a useful pedagogical device. That is, lots of problems are solved more easily by imagining the existence of an Ether.