The Collaborative International Dictionary of English v.0.48:

Remorse \Re*morse"\ (r?*m?rs"), n. [OE. remors, OF. remors,F.
   remords, LL. remorsus, fr. L. remordere, remorsum, to bite
   again or back, to torment; pref. re- re- + mordere to bite.
   See Morsel.]
   1. The anguish, like gnawing pain, excited by a sense of
      guilt; compunction of conscience for a crime committed, or
      for the sins of one's past life. "Nero will be tainted
      with remorse." --Shak.
      [1913 Webster]

   2. Sympathetic sorrow; pity; compassion.
      [1913 Webster]

            Curse on the unpardoning prince, whom tears can draw
            To no remorse.                        --Dryden.
      [1913 Webster]

            But evermore it seem'd an easier thing
            At once without remorse to strike her dead.
                                                  --Tennyson.
      [1913 Webster]

   Syn: Compunction; regret; anguish; grief; compassion. See
        Compunction.
        [1913 Webster]

WordNet (r) 3.0 (2006):

remorse
    n 1: a feeling of deep regret (usually for some misdeed) [syn:
         compunction, remorse, self-reproach]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "remorse":
   anxiety, apologies, attrition, ayenbite of inwit, bitterness,
   compunction, contriteness, contrition, embarrassment, grief, guilt,
   guilty conscience, humiliation, mortification, pangs of conscience,
   penance, penitence, penitently, regret, regretfulness, regrets,
   regretting, remorse of conscience, remorsefulness, repentance,
   repining, rue, ruefulness, self-reproach, shame, shamefacedness,
   shamefastness, shamefulness, sorriness, sorrow, wistfulness, woe