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]