The Collaborative International Dictionary of English v.0.48:

Benediction \Ben`e*dic"tion\, n. [L. benedictio: cf. F.
   b['e]n['e]diction. See Benison.]
   1. The act of blessing.
      [1913 Webster]

   2. A blessing; an expression of blessing, prayer, or kind
      wishes in favor of any person or thing; a solemn or
      affectionate invocation of happiness.
      [1913 Webster]

            So saying, he arose; whom Adam thus
            Followed with benediction.            --Milton.
      [1913 Webster]

            Homeward serenely she walked with God's benediction
            upon her.                             --Longfellow.
      [1913 Webster] Specifically: The short prayer which closes
      public worship; as, to give the benediction.
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   3. (Eccl.) The form of instituting an abbot, answering to the
      consecration of a bishop. --Ayliffe.
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   4. (R. C. Ch.) A solemn rite by which bells, banners,
      candles, etc., are blessed with holy water, and formally
      dedicated to God.
      [1913 Webster]

WordNet (r) 3.0 (2006):

benediction
    n 1: the act of praying for divine protection [syn: blessing,
         benediction]
    2: a ceremonial prayer invoking divine protection [syn:
       benediction, blessing]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "benediction":
   OK, acknowledgment, advantage, approval, benedicite, benefit,
   benison, blessing, boon, cognizance, credit, crediting, favor,
   godsend, grace, hymn, invocation, paean, praise, prayer of thanks,
   recognition, thank offering, thank-you, thanks, thanksgiving