The Collaborative International Dictionary of English v.0.48:

fierceness \fierceness\ n.
   the quality of being fierce; ferocity; fury; vehemence.

   Syn: ferocity, furiousness, fury, vehemence, violence,
        wildness, strength.
        [WordNet 1.5 +PJC]

The Collaborative International Dictionary of English v.0.48:

Fierce \Fierce\, a. [Compar. Fiercer; superl. Fiercest.]
   [OE. fers, fiers, OF. fier, nom. fiers, fierce, savage,
   cruel, F. fier proud, from L. ferus wild, savage, cruel;
   perh. akin to E. bear the animal. Cf. Feral, Ferocity.]
   1. Furious; violent; unrestrained; impetuous; as, a fierce
      wind.
      [1913 Webster]

            His fierce thunder drove us to the deep. --Milton.
      [1913 Webster]

   2. Vehement in anger or cruelty; ready or eager to kill or
      injure; of a nature to inspire terror; ferocious. "A
      fierce whisper." --Dickens. "A fierce tyrant." --Pope.
      [1913 Webster]

            The fierce foe hung upon our broken rear. --Milton.
      [1913 Webster]

            Thou huntest me as a fierce lion.     --Job. x. 16.
      [1913 Webster]

   3. Excessively earnest, eager, or ardent.

   Syn: Ferocious; savage; cruel; vehement; impetuous;
        barbarous; fell. See Ferocious. -- Fierce"ly, adv.
        -- Fierce"ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

fierceness
    n 1: the property of being wild or turbulent; "the storm's
         violence" [syn: ferocity, fierceness, furiousness,
         fury, vehemence, violence, wildness]