Source : Webster's Revised Unabridged Dictionary (1913)

Hoarfrost \Hoar"frost`\, n.
   The white particles formed by the congelation of dew; white
   frost. [Written also {horefrost}. See {Hoar}, a.]

         He scattereth the hoarfrost like ashes.  --Ps. cxlvii.
                                                  16.

Source : WordNet®

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