The Collaborative International Dictionary of English v.0.48:

Expiation \Ex`pi*a"tion\, n. [L. expiatio: cf.F. expiation]
   1. The act of making satisfaction or atonement for any crime
      or fault; the extinguishing of guilt by suffering or
      penalty.
      [1913 Webster]

            His liberality seemed to have something in it of
            self-abasement and expiation.         --W. Irving.
      [1913 Webster]

   2. The means by which reparation or atonement for crimes or
      sins is made; an expiatory sacrifice or offering; an
      atonement.
      [1913 Webster]

            Those shadowy expiations weak,
            The blood of bulls and goats.         --Milton.
      [1913 Webster]

   3. An act by which the threats of prodigies were averted
      among the ancient heathen. [Obs.] --Hayward.
      [1913 Webster]

WordNet (r) 3.0 (2006):

expiation
    n 1: compensation for a wrong; "we were unable to get
         satisfaction from the local store" [syn: atonement,
         expiation, satisfaction]
    2: the act of atoning for sin or wrongdoing (especially
       appeasing a deity) [syn: expiation, atonement,
       propitiation]