Source : Webster's Revised Unabridged Dictionary (1913)

Malignance \Ma*lig"nance\, Malignancy \Ma*lig"nan*cy\, n. [See
   {Malignant}.]
   1. The state or quality of being malignant; extreme
      malevolence; bitter enmity; malice; as, malignancy of
      heart.

   2. Unfavorableness; evil nature.

            The malignancy of my fate might perhaps distemner
            yours.                                --Shak.

   3. (Med.) Virulence; tendency to a fatal issue; as, the
      malignancy of an ulcer or of a fever.

   4. The state of being a malignant.

   Syn: Malice; malevolence; malignity. See {Malice}.

Source : WordNet®

malignancy
     n 1: (medicine) a malignant state; progressive and resistant to
          treatment and tending to cause death [syn: {malignance}]
     2: quality of being disposed to evil; intense ill will [syn: {malignity},
         {malignance}] [ant: {benignity}, {benignity}]