Sometimes spelled Ether, the term is now used only rarely (and not at all seriously) to describe the medium through which radio waves travel. Although it's been used for centuries in science, the term is now used in the radio community as more of a 'slang' word than anything else. For example, "That new transmitter and antenna may produce enough RF to burn holes in the ether!"
Here's some of the history of the term.....
In the 4th century BC, Aristotle theorized that the planets moved on a series of concentric spheres around the Earth. These spheres were composed of Aether, a name which originated with the Greeks, as they tried to describe the composition of the heavens. Aether was the 'shiny substance' of which the heavens were composed.
Descartes used the term to describe 'subtle matter', found between 'gross matter'.
Newton used it in theories on the cause of gravity, and attributed it as a source of 'fits' of refraction and reflection of light.
Aether became 'ethereal' - that is, it lost its substance - in the 17th century, and bordered upon the spiritual - in fact, earlier than this, in the late 1500's William Gilbert made reference to what he called 'Effluvium', which possessed many of the characteristics of 'Aether'. He theorized that it caused magnetism, and was the 'soul of the world' - actually possessing intelligence.
In the 18th Century, Aether went out of vogue, so to speak...and space was viewed as empty - other theories were developed to describe the action of the heavens and there was no need to speculate about an 'Aether' to explain the natural world.
In the 19th century, Thomas Young in England and Augustin Fresnel in France proposed wave theories of light. They supposed that a light wave must travel through something...and Aether must be that something.
Between 1850 and 1880, Michael Faraday and James Maxwell provided the basis for field theory. They needed something for the 'elastic strains' of a magnetic field to pass through, and be a part of. Aether, of course!
Albert Einstein destroyed the idea of an actual Aether with his Special Theory of Relativity in 1905. The presence of Aether, in the reality of this theory would have produced some rather unlikely results.
If you want more information on why, try this page...Hypertext Links on Relativity...cause you sure don't want to count on my explanation!