Source : Webster's Revised Unabridged Dictionary (1913)

Hydrocyanic \Hy`dro*cy*an"ic\, a. [Hydro-, 2 + anic: cf. F.
   hydrocyanique.] (Chem.)
   Pertaining to, or derived from the combination of, hydrogen
   and cyanogen.

   {Hydrocyanic acid} (Chem.), a colorless, mobile, volatile
      liquid, {HCN}, having a characteristic peach-blossom odor.
      It is one of the most deadly poisons. It is made by the
      action of sulphuric acid on yellow prussiate of potassium
      (potassium ferrocyanide), and chemically resembles
      hydrochloric and hydrobromic acids. Called also {prussic
      acid}, {hydrogen cyanide}, etc.

Source : WordNet®

hydrogen cyanide
     n : a highly poisonous gas or volatile liquid that smells like
         bitter almonds; becomes a gas at around 90 degree
         fahrenheit and is most dangerous when inhaled; the
         anhydride of hydrocyanic acid; used in manufacturing