The Collaborative International Dictionary of English v.0.48:

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; disposition toward
      evil; intense ill will; as, malignancy of heart.
      [1913 Webster]

   2. Unfavorableness; evil nature.
      [1913 Webster]

            The malignancy of my fate might perhaps distemner
            yours.                                --Shak.
      [1913 Webster]

   3. (Med.) Virulence; tendency to a fatal issue; as, the
      malignancy of an ulcer or of a fever.
      [1913 Webster]

   4. Hence: (Med.) A cancerous tumor that is spreading beyond
      the point of origin.

   Syn: malignant tumor, malignant neoplasm, metastatic tumor.
        [WordNet 1.5]

   5. The state of being a malignant.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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