The luminiferous ether was a substance conjectured to fill space, which acted as the medium through which radiation such as light passed. As light was considered a kind of wave motion it was assumed to need some kind of medium that it could cause to oscillate. As light travelled through space from the distant stars, space had to be filled with ether. Although now rejected, the idea was once taken very seriously by scientists.
The term drew on the ancient belief that the heavens were constructed from a quintessence, that is, a fifth element other than the four then considered to be the components of all materials on Earth (air, earth, fire and water), and known as ether or æther (aether).
The ether was assumed to be a very subtle fluid (as it did not impede the planets in their orbits) and the same name was also given to a volatile chemical compound (so there is a genuine substance called ether, although the luminiferous ether is no longer accepted).
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