The Collaborative International Dictionary of English v.0.48:

Ferrocyanic \Fer`ro*cy*an"ic\, a. [Ferro- + cyanic: cf. F.
   ferrocyanique.] (Chem.)
   Pertaining to, derived from, or designating, a ferrocyanide.
   [1913 Webster]

   ferrocyanic acid (Chem.), a white crystalline substance,
      H4(CN)6Fe, of strong acid properties, obtained from
      potassium ferrocyanide, and regarded as the type of the
      ferrocyanides; -- called also hydro-ferrocyanic acid,
      hydrogen ferrocyanide. etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ferrocyanic acid
    n 1: a white unstable acid formed from ferrocyanide salts