The Collaborative International Dictionary of English v.0.48:

Ethereal \E*the"re*al\, a.
   1. Pertaining to the hypothetical upper, purer air, or to the
      higher regions beyond the earth or beyond the atmosphere;
      celestial; as, ethereal space; ethereal regions.
      [1913 Webster]

            Go, heavenly guest, ethereal messenger. --Milton.
      [1913 Webster]

   2. Consisting of ether; hence, exceedingly light or airy;
      tenuous; spiritlike; characterized by extreme delicacy, as
      form, manner, thought, etc.
      [1913 Webster]

            Vast chain of being, which from God began,
            Natures ethereal, human, angel, man.  --Pope.
      [1913 Webster]

   3. (Chem.) Pertaining to, derived from, or resembling, ether;
      as, ethereal salts.
      [1913 Webster]

   Ethereal oil. (Chem.) See Essential oil, under
      Essential.

   Ethereal oil of wine (Chem.), a heavy, yellow, oily liquid
      consisting essentially of etherin, etherol, and ethyl
      sulphate. It is the oily residuum left after
      etherification. Called also heavy oil of wine
      (distinguished from oil of wine, or [oe]nanthic ether).

   Ethereal salt (Chem.), a salt of some organic radical as a
      base; an ester.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ethereal
    adj 1: characterized by lightness and insubstantiality; as
           impalpable or intangible as air; "figures light and
           aeriform come unlooked for and melt away"- Thomas
           Carlyle; "aerial fancies"; "an airy apparition";
           "physical rather than ethereal forms" [syn: aeriform,
           aerial, airy, aery, ethereal]
    2: of or containing or dissolved in ether; "ethereal solution"
    3: of heaven or the spirit; "celestial peace"; "ethereal
       melodies"; "the supernal happiness of a quiet death" [syn:
       celestial, ethereal, supernal]
    4: characterized by unusual lightness and delicacy; "this
       smallest and most ethereal of birds"; "gossamer shading
       through his playing" [syn: ethereal, gossamer]

Moby Thesaurus II by Grady Ward, 1.0:

229 Moby Thesaurus words for "ethereal":
   Elysian, Olympian, adulterated, aerial, aeriform, aerodynamic,
   aerostatic, aery, air-built, airish, airlike, airy, alfresco,
   altitudinous, ascending, asomatous, aspiring, astral, atmospheric,
   attenuate, attenuated, beatific, beatified, blessed, bodiless,
   boyish, breezy, bubbly, celestial, chimeric, chimerical,
   cloud-born, cloud-built, cloud-woven, colossal, cut, decarnate,
   decarnated, delicate, diaphanous, dilute, diluted, discarnate,
   disembodied, dominating, downy, dreamlike, ectoplasmic, elevated,
   eminent, empyreal, empyrean, etheric, exalted, exposed,
   extramundane, extraterrestrial, fanciful, fatuitous, fatuous,
   feathery, filmy, fine, fine-drawn, finespun, flimsy, fluffy, foamy,
   fragile, frail, from on high, frothy, fuming, fumy, gaseous,
   gasified, gasiform, gaslike, gassy, gauzy, ghostish, ghostlike,
   ghostly, ghosty, girlish, glorified, gossamer, gossamery, gracile,
   haughty, heavenly, high, high-pitched, high-reaching, high-set,
   high-up, illusory, imaginary, immaterial, impalpable, imponderable,
   imponderous, in glory, incorporate, incorporeal, insubstantial,
   intangible, lacy, leger, light, light as air, lighter than vanity,
   lofty, mephitic, miasmal, miasmatic, miasmic, misty, monumental,
   mounting, mousse, nonmaterial, nonphysical, occult, open-air,
   otherworldly, outtopping, overlooking, overtopping, oxyacetylene,
   oxygenous, ozonic, papery, paradisaic, paradisal, paradisiac,
   paradisic, phantasmal, phantasmic, phantom, phantomic, phantomlike,
   pneumatic, prominent, psychic, rare, rarefied, reeking, reeky,
   roomy, shadowy, slender, slenderish, slight, slight-made, slim,
   slimmish, slinky, small, smoking, smoky, soaring, souffle,
   specterlike, spectral, spiring, spiritual, spirituous, steaming,
   steamy, steep, sublime, subtile, subtle, superlative, supernal,
   supernatural, svelte, sylphlike, tenuous, thin, thin-bodied,
   thin-set, thin-spun, thinned, thinned-out, thinnish, threadlike,
   topless, toplofty, topping, towering, towery, transcendental,
   transmundane, tropospheric, uncompact, uncompressed, unearthly,
   unembodied, unextended, unfleshly, unheavy, unphysical, unreal,
   unsubstantial, unworldly, uplifted, upreared, vague, vaporing,
   vaporish, vaporlike, vaporous, vapory, volatile, wasp-waisted,
   watered, watered-down, watery, weak, weightless, willowy, windy,
   wiredrawn, wispy, wraithlike, wraithy, yeasty