The Collaborative International Dictionary of English v.0.48:

Penitent \Pen"i*tent\, n.
   1. One who repents of sin; one sorrowful on account of his
      transgressions.
      [1913 Webster]

   2. One under church censure, but admitted to penance; one
      undergoing penance.
      [1913 Webster]

   3. One under the direction of a confessor.
      [1913 Webster]

   Note: Penitents is an appellation given to certain
         fraternities in Roman Catholic countries, distinguished
         by their habit, and employed in charitable acts.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Penitent \Pen"i*tent\, a. [F. p['e]nitent, L. paenitens, -entis,
   poenitens, p. pr. of paenitere, poenitere, to cause to
   repent, to repent; prob. akin to poena punishment. See
   Pain.]
   1. Feeling pain or sorrow on account of sins or offenses;
      repentant; contrite; sincerely affected by a sense of
      guilt, and resolved on amendment of life.
      [1913 Webster]

            Be penitent, and for thy fault contrite. --Milton.
      [1913 Webster]

            The proud he tamed, the penitent he cheered.
                                                  --Dryden.
      [1913 Webster]

   2. Doing penance. [Obs.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

penitent
    adj 1: feeling or expressing remorse for misdeeds [syn:
           penitent, repentant] [ant: impenitent,
           unremorseful, unrepentant]
    n 1: (Roman Catholic Church) a person who repents for wrongdoing
         (a Roman Catholic may be admitted to penance under the
         direction of a confessor)

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "penitent":
   Magdalen, abject, apologetic, compunctious, confessor,
   conscience-stricken, contrite, grief-stricken, humbled, melted,
   penitential, penitentiary, prodigal returned, prodigal son,
   regretful, remorseful, repentant, rueful, sad, sheepish, softened,
   sorrowful, sorry, touched



The Devil's Dictionary (1881-1906):

PENITENT, adj.  Undergoing or awaiting punishment.