The Collaborative International Dictionary of English v.0.48:

Bitterness \Bit"ter*ness\, n. [AS. biternys; biter better + -nys
   = -ness.]
   1. The quality or state of being bitter, sharp, or acrid, in
      either a literal or figurative sense; implacableness;
      resentfulness; severity; keenness of reproach or sarcasm;
      deep distress, grief, or vexation of mind.
      [1913 Webster]

            The lip that curls with bitterness.   --Percival.
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            I will complain in the bitterness of my soul. --Job
                                                  vii. 11.
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   2. A state of extreme impiety or enmity to God.
      [1913 Webster]

            Thou art in the gall of bitterness, and in the bond
            of iniquity.                          --Acts viii.
                                                  23.
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   3. Dangerous error, or schism, tending to draw persons to
      apostasy.
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            Looking diligently, . . . lest any root of
            bitterness springing up trouble you.  --Heb. xii.
                                                  15.
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WordNet (r) 3.0 (2006):

bitterness
    n 1: a feeling of deep and bitter anger and ill-will [syn:
         resentment, bitterness, gall, rancor, rancour]
    2: a rough and bitter manner [syn: bitterness, acrimony,
       acerbity, jaundice, tartness, thorniness]
    3: the taste experience when quinine or coffee is taken into the
       mouth [syn: bitter, bitterness]
    4: the property of having a harsh unpleasant taste [syn:
       bitterness, bitter]