The Collaborative International Dictionary of English v.0.48:

Purgation \Pur*ga"tion\, n. [L. purgatio: cf. F. purgation. See
   Purge.]
   1. The act of purging; the act of clearing, cleansing, or
      putifying, by separating and carrying off impurities, or
      whatever is superfluous; the evacuation of the bowels.
      [1913 Webster]

   2. (Law) The clearing of one's self from a crime of which one
      was publicly suspected and accused. It was either
      canonical, which was prescribed by the canon law, the form
      whereof used in the spiritual court was, that the person
      suspected take his oath that he was clear of the matter
      objected against him, and bring his honest neighbors with
      him to make oath that they believes he swore truly; or
      vulgar, which was by fire or water ordeal, or by combat.
      See Ordeal. --Wharton.
      [1913 Webster]

            Let him put me to my purgation.       --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

purgation
    n 1: purging the body by the use of a cathartic to stimulate
         evacuation of the bowels [syn: catharsis, katharsis,
         purgation]
    2: a ceremonial cleansing from defilement or uncleanness by the
       performance of appropriate rites [syn: purification,
       purgation]
    3: the act of clearing yourself (or another) from some stigma or
       charge [syn: purge, purging, purgation]

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "purgation":
   BM, Day of Atonement, Yom Kippur, abreaction, absolution,
   abstersion, acquittal, acquittance, acting-out, asceticism,
   bloody flux, bowdlerization, bowel movement, catharsis, cleaning,
   cleansing, clearance, clearing, cold purgatorial fires,
   compurgation, crap, defecation, dejection, deliverance,
   destigmatization, destigmatizing, detersion, diarrhea, discharge,
   disculpation, dismissal, dry cleaning, dysentery,
   emotional release, evacuation, exculpation, excuse, exoneration,
   explanation, expurgation, fasting, flagellation, flux, forgiveness,
   freeing, hair shirt, intermission, justification, lientery,
   loose bowels, lustration, maceration, mortification,
   motor abreaction, movement, outlet, pardon, penance, penitence,
   penitential act, penitential exercise, psychocatharsis,
   psychodrama, purgatory, purge, purging, purification, quietus,
   quittance, rationalization, rehabilitation, reinstatement, release,
   release therapy, remission, removal, repentance, reprieve, respite,
   restoration, runs, sackcloth and ashes, shit, shits,
   steam cleaning, stool, surcease, suspension, trots, turistas,
   verdict of acquittal, vindication, voidance



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

PURGATION. The clearing one's self of an offence charged, by denying the
guilt on oath or affirmation.
     2. There were two sorts of purgation, the vulgar, and the canonical.
     3. Vulgar purgation consisted in superstitious trials by hot and cold
water, by fire, by hot irons, by batell, by corsned, &c., which modes of
trial were adopted in times of ignorance and barbarity, and were impiously
called judgments of God.
     4. Canonical purgation was the act of justifying one's self, when
accused of some offence in the presence of a number of persons, worthy of
credit, generally twelve, who would swear they believed the accused. See
Compurgator; Wager of Law.
     5. In modern times, a man may purge himself of an offence, in some
cases where the facts are within his own knowledge; for example, when a man
is charged with a contempt of court, he may purge himself of such contempt,
by swearing that in doing the act charged, he did not intend to commit a
contempt.