Luminiferous ether

In the late 19th century, luminiferous aether, æther or ether, meaning light-bearing aether, was the hypothetical medium for the propagation of light and heat, filling all unoccupied space.

In Newtonian physics all waves are propagated through a medium, e.g., water waves through water, sound waves through air. Ether was held to be invisible, without odor, and of such a nature that it did not interfere with the motions of bodies through space. However, all attempts to demonstrate its existence produced negative results and stimulated a vigorous debate among physicists that was not ended until the special theory of relativity, proposed by Albert Einstein in 1905, became accepted. The theory of relativity eliminated the need for a light-transmitting medium, so that today the term ether is used only in a historical context.

(Source: Wolfram Research World of Physics, Image—Doug Wheeler 1973)