The Collaborative International Dictionary of English v.0.48:

Thanksgiving \Thanks"giv`ing\, n.
   1. The act of rending thanks, or expressing gratitude for
      favors or mercies.
      [1913 Webster]

            Every creature of God is good, and nothing to be
            refused, if it be received with thanksgiving. --1
                                                  Tim. iv. 4.
      [1913 Webster]

            In the thanksgiving before meat.      --Shak.
      [1913 Webster]

            And taught by thee the Church prolongs
            Her hymns of high thanksgiving still. --Keble.
      [1913 Webster]

   2. A public acknowledgment or celebration of divine goodness;
      also, a day set apart for religious services, specially to
      acknowledge the goodness of God, either in any remarkable
      deliverance from calamities or danger, or in the ordinary
      dispensation of his bounties.
      [1913 Webster]

   Note: In the United States it is now customary for the
         President by proclamation to appoint annually a day
         (usually the last Thursday in November) of thanksgiving
         and praise to God for the mercies of the past year.
         This is an extension of the custom long prevailing in
         several States in which an annual Thanksgiving day has
         been appointed by proclamation of the governor.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Thanksgiving
    n 1: fourth Thursday in November in the United States; second
         Monday in October in Canada; commemorates a feast held in
         1621 by the Pilgrims and the Wampanoag [syn:
         Thanksgiving, Thanksgiving Day]
    2: a short prayer of thanks before a meal; "their youngest son
       said grace" [syn: grace, blessing, thanksgiving]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "thanksgiving":
   Angelus, Ave, Ave Maria, Hail Mary, Kyrie Eleison, Paternoster,
   acknowledgment, aid prayer, appeal, beadroll, beads, benediction,
   beseechment, bidding prayer, blessing, breviary, chaplet,
   cognizance, collect, communion, contemplation, credit, crediting,
   devotions, entreaty, grace, hymn, impetration, imploration,
   intercession, invocation, litany, meditation, obsecration,
   obtestation, orison, paean, petition, praise, prayer,
   prayer of thanks, prayer wheel, recognition, rogation, rosary,
   silent prayer, suit, supplication, thank offering, thank-you,
   thanks