Ether, or luminiferous Ether, was the hypothetical substance through which electromagnetic waves travel. It was proposed by the greek philosopher Aristotle and used by several optical theories as a way to allow propagation of light, which was believed to be impossible in "empty" space.

It was supposed that the ether filled the whole universe and was a stationary frame of reference, which was rigid to electromagnetic waves but completely permeable to matter. Hooke endorsed the idea of the existence of the ether in his work Micrographia (1665), and other several philosophers of the 17th century, including Huygens,did the same. At the time of Maxwell's mathematical studies of electromagnetism, ether was still assumed to be the propagation medium and was imbued with physics properties such as permeability and permittivity.

In 1887, a crucial experiment was performed by Michelson and Edward Morley in an attempt to detect the existence of the ether. The experiment, named the Michelson-Morley experiment in honor of its authors, shocked the scientific community by yielding results which implied the non-existence of ether. This result was later on used by Einstein to refute the existence of the ether and allowed him to develop special relativity without this artificial (and non-existent) constraint.