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From The Collaborative International Dictionary of English v.0.48:
Ether \E"ther\ ([=e]"th[~e]r), n. [L. aether, Gr. a'iqh`r, fr. a'i`qein to light up, kindle, burn, blaze; akin to Skr. idh, indh, and prob. to E. idle: cf. F. ['e]ther.] [Written also [ae]ther.] 1. (Physics) A medium of great elasticity and extreme tenuity, once supposed to pervade all space, the interior of solid bodies not excepted, and to be the medium of transmission of light and heat; hence often called luminiferous ether. It is no longer believed that such a medium is required for the transmission of electromagnetic waves; the modern use of the term is mostly a figurative term for empty space, or for literary effect, and not intended to imply the actual existence of a physical medium. However. modern cosmological theories based on quantum field theory do not rule out the possibility that the inherent energy of the vacuum is greater than zero, in which case the concept of an ether pervading the vacuum may have more than metaphoric meaning. [1913 Webster +PJC] 2. Supposed matter above the air; the air itself. [1913 Webster] 3. (Chem.) (a) A light, volatile, mobile, inflammable liquid, (C2H5)2O, of a characteristic aromatic odor, obtained by the distillation of alcohol with sulphuric acid, and hence called also sulphuric ether. It is a powerful solvent of fats, resins, and pyroxylin, but finds its chief use as an an[ae]sthetic. Commonly called ethyl ether to distinguish it from other ethers, and also ethyl oxide. (b) Any similar compound in which an oxygen atom is bound to two different carbon atoms, each of which is part of an organic radical; as, amyl ether; valeric ether; methyl ethyl ether. The general formular for an ether is ROR', in which R and R' are organic radicals which may be of similar or different structure. If R and R' are different parts of the same organic radical, the structure forms a cyclic ether. [1913 Webster +PJC] Complex ether, Mixed ether (Chem.), an ether in which the ether oxygen is attached to two radicals having different structures; as, ethyl methyl ether, C2H5.O.CH3. Compound ether (Chem.), an ethereal salt or a salt of some hydrocarbon as the base; an ester. Ether engine (Mach.), a condensing engine like a steam engine, but operated by the vapor of ether instead of by steam. [1913 Webster]