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]

WordNet (r) 3.0 (2006):

ethyl ether
    n 1: a colorless volatile highly inflammable liquid formerly
         used as an inhalation anesthetic [syn: ether,
         ethoxyethane, divinyl ether, vinyl ether, diethyl
         ether, ethyl ether]