Ether, sometimes called "luminiferous ether," is the fifth essence upon which Reality is founded. The term derives from the substance which was once theorized to be the medium through which light propagates in space. The Technocracy eliminated ether from the Consensus in 1905, driving the Electrodyne Engineers to leave the Union and join the Council of Nine Mystic Traditions as the Sons of Ether.
The Kitab al-Alacir or Book of Ether was purportedly written by a Trojan mage named Aretus, laying the foundation for the study of ether by Western philosophers. Aristotle proposed that, in addition to the four classical elements of earth, air, water and fire, a fifth element with unique properties existed that explained the motion of stars and planets. This fifth element, called ether or quintessence, was heavily studied by Sleeper alchemists. Meanwhile, among the Awakened of the 12th century, the original text of the Kitab al-Alacir was rediscovered (in its Arabic translation) and the Natural Philosopher's Guild was formed to study ether as a metaphysical and physical phenomenon. 
In the 17th century, Sleeper scientists theorized that something had to exist to allow light, heat, magnetism and other forces to operate on objects at a distance. Just as sound is a pressure wave propagating through air, they reasoned, these forces were propagating through some medium of their own. This medium was called luminiferous ether or aether, and scientists wrangled with one another over its nature and properties: on one hand, it had to have many of the mechanical characteristics of matter, but on the other, it couldn't be shown to ever interact with other forms of matter. Experiments designed to detect "ripples" in the ether were either negative or inconclusive. 
The Electrodyne Engineers, Technocratic successors of the Natural Philosopher's Guild, understood the limitations of these experiments due to their own research guided by the Kitab al-Alacir. Members such as Nikola Tesla were working towards a unified theory of energy with the potential to revolutionize technology. Alarmed at the pace of these developments, the Technocratic Union ruined Tesla's career and encouraged Albert Einstein to exclude the notion of ether from his work on relativity. Thus ether was eliminated from the Consensus, triggering the defection of the Engineers to the Traditions.
According to the paradigm of the Sons of Ether, True Ether underpins the entire universe and unites a variety of disparate phenomena. All natural forces are simply difference frequencies of vibration propagating through True Ether. They consider Quintessence and the nine Spheres to be various manifestations of ether, with True Ether itself as the Tenth Sphere that will explain the fundamental nature of the universe.