The Collaborative International Dictionary of English v.0.48:

Yom Kippur \Yom` Kip*pur"\ (y[=o]m`k[i^]*p[oo^]r",
   y[aum]m`k[i^]*p[oo^]r"), [Heb. y[=o]m kipp[=u]r, day of
   atonement.] (Jewish Antiq.)
   the only fast day of the Mosaic ritual, celebrated on the
   tenth day of the seventh month (Tishri), according to the
   rites described in Leviticus xvi. Also called Day of
   Atonement.
   [Webster 1913 Suppl. +PJC]

The Collaborative International Dictionary of English v.0.48:

Atonement \A*tone"ment\, n.
   1. (Literally, a setting at one.) Reconciliation; restoration
      of friendly relations; agreement; concord. [Archaic]
      [1913 Webster]

            By whom we have now received the atonement. --Rom.
                                                  v. 11.
      [1913 Webster]

            He desires to make atonement
            Betwixt the Duke of Gloucester and your brothers.
                                                  --Shak.
      [1913 Webster]

   2. Satisfaction or reparation made by giving an equivalent
      for an injury, or by doing of suffering that which will be
      received in satisfaction for an offense or injury;
      expiation; amends; -- with for. Specifically, in theology:
      The expiation of sin made by the obedience, personal
      suffering, and death of Christ.
      [1913 Webster]

            When a man has been guilty of any vice, the best
            atonement be can make for it is, to warn others.
                                                  --Spectator.
      [1913 Webster]

            The Phocians behaved with, so much gallantry, that
            they were thought to have made a sufficient
            atonement for their former offense.   --Potter.
      [1913 Webster]

   Day of Atonement (Jewish Antiq.), the only fast day of the
      Mosaic ritual, celebrated on the tenth day of the seventh
      month (Tishri), according to the rites described in
      Leviticus xvi. Also called Yom Kippur.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Day of Atonement
    n 1: (Judaism) a solemn and major fast day on the Jewish
         calendar; 10th of Tishri; its observance is one of the
         requirements of the Mosaic law [syn: Yom Kippur, Day of
         Atonement]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "Day of Atonement":
   Fast of Av, Feast of Tabernacles, Feast of Weeks, Hanukkah,
   High Holy Days, New Year, Ninth of Av, Passover, Pentecost, Pesach,
   Purim, Rosh Hashanah, Shabuoth, Simhath Torah, Sukkoth, Yom Kippur,
   asceticism, cold purgatorial fires, fasting, flagellation,
   hair shirt, lustration, maceration, mortification, penance,
   penitence, penitential act, penitential exercise, purgation,
   purgatory, repentance, sackcloth and ashes