noun

1. a substance hypothesized by the ancients as filling all space beyond the sphere of the moon, and making up the stars and planets
2. the upper regions of space; clear sky
3. Rare the air
4. Chem. any of a series of organic compounds having an oxygen atom linking two carbon atoms from two hydrocarbon radicals, generally ROR; esp., ethyl ether, a volatile, colorless, highly flammable liquid, HCOCH, prepared by the reaction of sulfuric acid and ethyl alcohol and used as an anesthetic and a solvent for resins and fats
5. Physics a hypothetical diffuse, invisible medium formerly thought to pervade space and transmit light, heat, etc.

Etymology: ME < L aether < Gr aithēr < aithein, to kindle, burn < IE base *aidh- > L aestas, summer, OE ætan, to burn