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]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "expiation":
   amends, atonement, balancing, commutation, compensation,
   composition, compromise, counteraction, counterbalancing,
   expiatory offering, indemnification, indemnity, lex talionis,
   making amends, making good, making right, making up, offsetting,
   peace offering, piaculum, propitiation, quittance, reclamation,
   recompense, rectification, redemption, redress, reparation,
   repayment, restitution, retaliation, revenge, satisfaction,
   squaring, substitution



Easton's 1897 Bible Dictionary:

Expiation
   Guilt is said to be expiated when it is visited with punishment
   falling on a substitute. Expiation is made for our sins when
   they are punished not in ourselves but in another who consents
   to stand in our room. It is that by which reconciliation is
   effected. Sin is thus said to be "covered" by vicarious
   satisfaction.

     The cover or lid of the ark is termed in the LXX. hilasterion,
   that which covered or shut out the claims and demands of the law
   against the sins of God's people, whereby he became "propitious"
   to them.

     The idea of vicarious expiation runs through the whole Old
   Testament system of sacrifices. (See PROPITIATION.)