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.
      [1913 Webster]

            I will complain in the bitterness of my soul. --Job
                                                  vii. 11.
      [1913 Webster]

   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.
      [1913 Webster]

   3. Dangerous error, or schism, tending to draw persons to
      apostasy.
      [1913 Webster]

            Looking diligently, . . . lest any root of
            bitterness springing up trouble you.  --Heb. xii.
                                                  15.
      [1913 Webster]

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]

Moby Thesaurus II by Grady Ward, 1.0:

172 Moby Thesaurus words for "bitterness":
   absolute zero, acerbity, aching heart, acid, acidity, acidness,
   acidulousness, acridity, acridness, acrimony, agony, agony of mind,
   algidity, anguish, animosity, animus, antagonism, apologies,
   asperity, astringence, astringency, attrition, ayenbite of inwit,
   bad blood, bale, bile, bite, bitingness, bitter feeling,
   bitter pill, bitter resentment, bitterness, bitterness of spirit,
   bleakness, bleeding heart, briskness, broken heart, causticity,
   causticness, cheerlessness, chill, chilliness, choler, cold,
   coldness, comfortlessness, contriteness, contrition, cool,
   coolness, coolth, corrosiveness, crispness, crushing, cryogenics,
   cryology, cuttingness, decrease in temperature, depression,
   depth of misery, desolation, despair, discomfort, dismalness,
   distress, distressfulness, dreariness, edge, enmity, extremity,
   feud, fierceness, freezing point, freshness, frigidity, frostiness,
   gall, gall and wormwood, gelidity, gnashing of teeth, grief,
   grievousness, grip, hard feelings, harshness, hatred, heartache,
   heartburning, heavy heart, hostility, iciness, ill blood,
   ill feeling, ill will, incisiveness, inclemency, infelicity,
   intense cold, joylessness, keenness, lamentability, lamentation,
   low temperature, melancholia, melancholy, misery, mordacity,
   mordancy, mournfulness, nip, nippiness, pain, painfulness, pathos,
   piercingness, piquancy, pitiability, pitiableness, pitifulness,
   poignancy, point, prostration, pungency, rancor, rankling, rawness,
   regret, regretfulness, regrets, regrettableness, regretting,
   remorse, remorse of conscience, remorsefulness, repining,
   resentment, rigor, roughness, sadness, severity, shame,
   shamefacedness, shamefastness, shamefulness, sharp air, sharpness,
   slow burn, soreness, sorriness, sorrow, sorrowfulness, sourness,
   spleen, stabbingness, sting, stridency, stringency,
   suicidal despair, tartness, teeth, trenchancy, vehemence, vendetta,
   venom, violence, virulence, vitriol, wistfulness, woe,
   woebegoneness, woefulness, wretchedness