The Collaborative International Dictionary of English v.0.48:

Propitiation \Pro*pi`ti*a"tion\, n. [L. propitiatio: cf. F.
   propitiation.]
   [1913 Webster]
   1. The act of appeasing the wrath and conciliating the favor
      of an offended person; the act of making propitious.
      [1913 Webster]

   2. (Theol.) That which propitiates; atonement or atoning
      sacrifice; specifically, the influence or effects of the
      death of Christ in appeasing the divine justice, and
      conciliating the divine favor.
      [1913 Webster]

            He [Jesus Christ] is the propitiation for our sins.
                                                  --1 John ii.
                                                  2.
      [1913 Webster]

WordNet (r) 3.0 (2006):

propitiation
    n 1: the act of placating and overcoming distrust and animosity
         [syn: placation, conciliation, propitiation]
    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:

42 Moby Thesaurus words for "propitiation":
   United Nations troops, amends, appeasement, atonement, calming,
   compensation, composition, compromise, conciliation, detente,
   dulcification, easing of relations, expiation, expiatory offering,
   indemnification, indemnity, making amends, making good,
   making right, making up, mediation, mollification, pacification,
   peace offering, peace-keeping force, peacemaking, peacemongering,
   piaculum, placability, placation, quittance, reclamation,
   recompense, redemption, redress, relaxation of tension, reparation,
   restitution, satisfaction, soothing, squaring, tranquilization



Easton's 1897 Bible Dictionary:

Propitiation
   that by which God is rendered propitious, i.e., by which it
   becomes consistent with his character and government to pardon
   and bless the sinner. The propitiation does not procure his love
   or make him loving; it only renders it consistent for him to
   execise his love towards sinners.

     In Rom. 3:25 and Heb. 9:5 (A.V., "mercy-seat") the Greek word
   _hilasterion_ is used. It is the word employed by the LXX.
   translators in Ex. 25:17 and elsewhere as the equivalent for the
   Hebrew _kapporeth_, which means "covering," and is used of the
   lid of the ark of the covenant (Ex. 25:21; 30:6). This Greek
   word (hilasterion) came to denote not only the mercy-seat or lid
   of the ark, but also propitation or reconciliation by blood. On
   the great day of atonement the high priest carried the blood of
   the sacrifice he offered for all the people within the veil and
   sprinkled with it the "mercy-seat," and so made propitiation.

     In 1 John 2:2; 4:10, Christ is called the "propitiation for
   our sins." Here a different Greek word is used (hilasmos).
   Christ is "the propitiation," because by his becoming our
   substitute and assuming our obligations he expiated our guilt,
   covered it, by the vicarious punishment which he endured. (Comp.
   Heb. 2:17, where the expression "make reconciliation" of the
   A.V. is more correctly in the R.V. "make propitiation.")