Or, more properly, the luminiferous aether (or ether, if you prefer). The luminiferous aether is that odd substance that fills the spaces between the planets, a fluid medium which carries lightwaves from the sun through space.
The luminiferous aether was first theorized by Isaac Newton in 1704's Opticks. He claimed it was required for the effect of light refraction. In 1818, a French physicist named Augustin Fresnel proposed a theory that light was a transverse wave like sound, which would require a medium for propagation of the wave. That medium was 'the aether'. From this point on, until the late 1880s, no one disputes the existence of the aether, they merely have difficulty defining how it operates, and different theories about that are tested with mixed results.
In Victorian Science Fiction, the luminiferous aether (usually abbreviated to simply 'the aether') is frequently used as a means of carrying vessels between the planets of our solar system. How this takes place varies from author to author, or gamer to gamer, as the case may be. Personally, I combine the ideas of Frank Chadwick's Space 1889 setting (a manipulation of electromagnetics to provide propulsion, the so-called 'aether propeller') and a sort of cold nuclear reactor, powered by a miraculous (and fictional) mineral I call 'handwavium.' Handwavium produces an intense heat via chemical reaction with acids, like vinegar. The acid is slowly consumed, but the process uses no oxygen, making it perfect for use in a boiler in space.