Æther, or Ether, is a historical concept that refers to a substance once thought to fill the universe beyond the terrestrial sphere. In ancient and medieval philosophy, it was considered the fifth element, alongside earth, water, air, and fire, believed to be the medium through which light and celestial bodies moved. In the 19th century, scientists proposed the existence of luminiferous ether as a medium for the propagation of light waves. However, experiments, such as the famous Michelson-Morley experiment, failed to detect it, leading to its eventual dismissal in favor of the theory of relativity proposed by Albert Einstein.