noun Latin Fullness of matter in space.
A space completely filled with matter, or the whole of space so regarded.
Lord Kelvin writing in 1893, in his preface to the English edition of Hertz's Researches on Electric Waves, says "many workers and many thinkers have helped to build up the nineteenth century school of plenum one ether for light, heat, electricity, magnetism; and the German and English volumes containing Hertz's electrical papers, given to the world in the last decade of the century, will be a permanent monument of the splendid consummation now realised." [ Saha & Bose, The Principle of Relativity, 1920, page i]
In the late 19th century, luminiferous aether or ether ("luminiferous", meaning "light-bearing"), was the postulated medium for the propagation of light. It was invoked to explain the ability of the apparently wave-based light to propagate through empty space, something that waves should not be able to do. The assumption of a spatial plenum of luminiferous aether, rather than a spatial vacuum, provided the theoretical medium that was required by wave theories of light.
The concept was the topic of considerable debate throughout its history, as it required the existence of an invisible and infinite material with no interaction with physical objects. As the nature of light was explored, especially in the 19th century, the physical qualities required of the ether became increasingly contradictory. By the late 1800s, the existence of the ether was being questioned, although there was no physical theory to replace it.
The negative outcome of the Michelson–Morley experiment (1887) suggested that the ether was non-existent, findings which were confirmed in subsequent experiments through the 1920s. This led to considerable theoretical work to explain the propagation of light without an ether. A major breakthrough was the theory of relativity, which could explain why the experiment failed to see ether, but was more broadly interpreted to suggest that it was not needed. The Michelson-Morley experiment, along with the blackbody radiator and photoelectric effect, was a key experiment in the development of modern physics, which includes both relativity and quantum theory, the latter of which explains the wave-like nature of light. Wikipedia, Luminiferous Ether