A medium that was once supposed to fill all space and to support the propagation of electromagnetic waves WordNet 3.0 & Open English WordNet

Often as aether and more fully as luminiferous aether: a substance once thought to fill all unoccupied space that allowed electromagnetic waves to pass through it and interact with matter, without exerting any resistance to matter or energy; its existence was disproved by the 1887 Michelson–Morley experiment and the theory of relativity propounded by Albert Einstein (1879–1955). Wiktionary

Substance once thought to fill all unoccupied space. Wiktionary (translation)