The Collaborative International Dictionary of English v.0.48:

Penance \Pen"ance\, v. t. [imp. & p. p. Penanced.]
   To impose penance; to punish. "Some penanced lady elf."
   --Keats.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Penance \Pen"ance\, n. [OF. penance, peneance, L. paenitentia
   repentance. See Penitence.]
   1. Repentance. [Obs.] --Wyclif (Luke xv. 7).
      [1913 Webster]

   2. Pain; sorrow; suffering. [Obs.] "Joy or penance he feeleth
      none." --Chaucer.
      [1913 Webster]

   3. (Eccl.) A means of repairing a sin committed, and
      obtaining pardon for it, consisting partly in the
      performance of expiatory rites, partly in voluntary
      submission to a punishment corresponding to the
      transgression, imposed by a confessor or other
      ecclesiastical authority. Penance is the fourth of seven
      sacraments in the Roman Catholic Church. --Schaff-Herzog
      Encyc.
      [1913 Webster]

            And bitter penance, with an iron whip. --Spenser.
      [1913 Webster]

            Quoth he, "The man hath penance done,
            And penance more will do."            --Coleridge.
      [1913 Webster]

   4. Hence: Any act performed by a person to atone for an
      offense to another; an act of atonement. [Colloq.]
      [PJC]

WordNet (r) 3.0 (2006):

penance
    n 1: remorse for your past conduct [syn: repentance,
         penitence, penance]
    2: a Catholic sacrament; repentance and confession and atonement
       and absolution
    3: voluntary self-punishment in order to atone for some
       wrongdoing [syn: penance, self-mortification, self-
       abasement]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "penance":
   Day of Atonement, Yom Kippur, abject apology, amends, apology,
   asceticism, atone, atonement, attrition, baptism, change of heart,
   cold purgatorial fires, compensation, compunction, confirmation,
   contrition, deathbed repentance, do penance, extreme unction,
   fasting, flagellation, hair shirt, heartfelt apology, holy orders,
   lustration, maceration, make amends, matrimony, mea culpa,
   mortification, pay, penal retribution, penalization, penalty,
   penitence, penitential act, penitential exercise, penitently,
   price, punishment, purgation, purgatory, reformation, regret,
   remorse, remorsefulness, reparation, repentance, rue, ruth,
   sackcloth and ashes, saeta, self-mortification, self-punishment,
   seven sacraments, suffer, suffering, the Eucharist,
   wearing a hairshirt



Bouvier's Law Dictionary, Revised 6th Ed (1856):

PENANCE, eccl. law. An ecclesiastical punishment, inflicted by an
ecclesiastical court, for some spiritual offence. Ayl. Par. 420.