Thayer's Greek Lexicon

 1. relating to an appeasing or expiating, having placating or
expiating force, expiatory; a means of appeasing or expiating,
a propitiation
  1) used of the cover of the ark of the covenant in the Holy
of Holies, which was sprinkled with the blood of the
expiatory victim on the annual day of atonement (this rite
signifying that the life of the people, the loss of which
they had merited by their sins, was offered to God in the
blood as the life of the victim, and that God by this
ceremony was appeased and their sins expiated); hence the
lid of expiation, the propitiatory
  2) an expiatory sacrifice
  3) a expiatory victim