The Collaborative International Dictionary of English v.0.48:

Vapor \Va"por\, n. [OE. vapour, OF. vapour, vapor, vapeur, F.
   vapeur, L. vapor; probably for cvapor, and akin to Gr. ?
   smoke, ? to breathe forth, Lith. kvepti to breathe, smell,
   Russ. kopote fine soot. Cf. Vapid.] [Written also
   vapour.]
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   1. (Physics) Any substance in the gaseous, or aeriform,
      state, the condition of which is ordinarily that of a
      liquid or solid.
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   Note: The term vapor is sometimes used in a more extended
         sense, as identical with gas; and the difference
         between the two is not so much one of kind as of
         degree, the latter being applied to all permanently
         elastic fluids except atmospheric air, the former to
         those elastic fluids which lose that condition at
         ordinary temperatures. The atmosphere contains more or
         less vapor of water, a portion of which, on a reduction
         of temperature, becomes condensed into liquid water in
         the form of rain or dew. The vapor of water produced by
         boiling, especially in its economic relations, is
         called steam.
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               Vapor is any substance in the gaseous condition
               at the maximum of density consistent with that
               condition. This is the strict and proper meaning
               of the word vapor.                 --Nichol.
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   2. In a loose and popular sense, any visible diffused
      substance floating in the atmosphere and impairing its
      transparency, as smoke, fog, etc.
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            The vapour which that fro the earth glood [glided].
                                                  --Chaucer.
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            Fire and hail; snow and vapors; stormy wind
            fulfilling his word.                  --Ps. cxlviii.
                                                  8.
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   3. Wind; flatulence. [Obs.] --Bacon.
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   4. Something unsubstantial, fleeting, or transitory; unreal
      fancy; vain imagination; idle talk; boasting.
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            For what is your life? It is even a vapor, that
            appeareth for a little time, and then vanisheth
            away.                                 --James iv.
                                                  14.
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   5. pl. An old name for hypochondria, or melancholy; the
      blues. "A fit of vapors." --Pope.
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   6. (Pharm.) A medicinal agent designed for administration in
      the form of inhaled vapor. --Brit. Pharm.
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   Vapor bath.
      (a) A bath in vapor; the application of vapor to the body,
          or part of it, in a close place; also, the place
          itself.
      (b) (Chem.) A small metallic drying oven, usually of
          copper, for drying and heating filter papers,
          precipitates, etc.; -- called also air bath. A
          modified form is provided with a jacket in the outside
          partition for holding water, or other volatile liquid,
          by which the temperature may be limited exactly to the
          required degree.

   Vapor burner, a burner for burning a vaporized hydrocarbon.


   Vapor density (Chem.), the relative weight of gases and
      vapors as compared with some specific standard, usually
      hydrogen, but sometimes air. The vapor density of gases
      and vaporizable substances as compared with hydrogen, when
      multiplied by two, or when compared with air and
      multiplied by 28.8, gives the molecular weight.

   Vapor engine, an engine worked by the expansive force of a
      vapor, esp. a vapor other than steam.
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WordNet (r) 3.0 (2006):

vapor density
    n 1: the density of a gas relative to the density of hydrogen
         [syn: vapor density, vapour density]