The term aether (or ether) originates from the ancient Greeks. It meant the upper
Later on the term was used by physicists to refer to a describe a substance, medium, or field, that fills all of space, and through which certains waves or forces can propagate. It was used to overcome apparent problems in the known theories. For example, in Newton's theory of gravity the gravitational force acts instantaneously and at a distance. This seems paradoxical, and against common sense. One solution to this problem is to introduce a medium, the gravitational aether, through which the gravitational force propagates. Now this medium is the one to exert the force and not the actual object, so there is no more spooky action at a distance.
The most famous theory involving an aether was the luminiferous aether - the medium through which light traveled. The theory was abandoned, along with the term, in the early 20th century, when it was replaced with Einstein's theories of relativity and concepts like space-time and quantum fields.