The Collaborative International Dictionary of English v.0.48:

Frost \Frost\ (fr[o^]st; 115), n. [OE. frost, forst, AS. forst,
   frost. fr. fre['o]san to freeze; akin to D. varst, G., OHG.,
   Icel., Dan., & Sw. frost. [root]18. See Freeze, v. i.]
   1. The act of freezing; -- applied chiefly to the congelation
      of water; congelation of fluids.
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   2. The state or temperature of the air which occasions
      congelation, or the freezing of water; severe cold or
      freezing weather.
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            The third bay comes a frost, a killing frost.
                                                  --Shak.
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   3. Frozen dew; -- called also hoarfrost or white frost.
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            He scattereth the hoarfrost like ashes. --Ps.
                                                  cxlvii. 16.
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   4. Coldness or insensibility; severity or rigidity of
      character. [R.]
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            It was of those moments of intense feeling when the
            frost of the Scottish people melts like a snow
            wreath.                               --Sir W.
                                                  Scott.
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   Black frost, cold so intense as to freeze vegetation and
      cause it to turn black, without the formation of
      hoarfrost.

   Frost bearer (Physics), a philosophical instrument
      illustrating the freezing of water in a vacuum; a
      cryophorus.

   Frost grape (Bot.), an American grape, with very small,
      acid berries.

   Frost lamp, a lamp placed below the oil tube of an Argand
      lamp to keep the oil limpid on cold nights; -- used
      especially in lighthouses. --Knight.

   Frost nail, a nail with a sharp head driven into a horse's
      shoe to keep him from slipping.

   Frost smoke, an appearance resembling smoke, caused by
      congelation of vapor in the atmosphere in time of severe
      cold.
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            The brig and the ice round her are covered by a
            strange black
            obscurity: it is the frost smoke of arctic winters.
                                                  --Kane.

   Frost valve, a valve to drain the portion of a pipe,
      hydrant, pump, etc., where water would be liable to
      freeze.

   Jack Frost, a popular personification of frost.
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The Collaborative International Dictionary of English v.0.48:

Hoarfrost \Hoar"frost`\, n.
   The white particles formed by the congelation of dew; white
   frost. [Written also horefrost. See Hoar, a.]
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         He scattereth the hoarfrost like ashes.  --Ps. cxlvii.
                                                  16.
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WordNet (r) 3.0 (2006):

hoarfrost
    n 1: ice crystals forming a white deposit (especially on objects
         outside) [syn: frost, hoar, hoarfrost, rime]