The Collaborative International Dictionary of English v.0.48:

thank \thank\ (th[a^][ng]k), n.; pl. thanks (th[a^][ng]ks).
   [AS. [thorn]anc, [thorn]onc, thanks, favor, thought; akin to
   OS. thank favor, pleasure, thanks, D. & G. dank thanks, Icel.
   [thorn]["o]kk, Dan. tak, Sw. tack, Goth. [thorn]agks thanks;
   -- originally, a thought, a thinking. See Think.]
   A expression of gratitude; an acknowledgment expressive of a
   sense of favor or kindness received; obligation, claim, or
   desert, or gratitude; -- now generally used in the plural.
   "This ceremonial thanks." --Massinger.
   [1913 Webster]

         If ye do good to them which do good to you, what thank
         have ye? for sinners also do even the same. --Luke vi.
                                                  33.
   [1913 Webster]

         What great thank, then, if any man, reputed wise and
         constant, will neither do, nor permit others under his
         charge to do, that which he approves not, especially in
         matter of sin?                           --Milton.
   [1913 Webster]

         Thanks, thanks to thee, most worthy friend,
         For the lesson thou hast taught.         --Longfellow.
   [1913 Webster]

   His thanks, Her thanks, etc., of his or her own accord;
      with his or her good will; voluntary. [Obs.]
      [1913 Webster]

            Full sooth is said that love ne lordship,
            Will not, his thanks, have no fellowship. --Chaucer.
      [1913 Webster]

   In thank, with thanks or thankfulness. [Obs.]

   Thank offering, an offering made as an expression of
      thanks.
      [1913 Webster]

WordNet (r) 3.0 (2006):

thanks
    n 1: an acknowledgment of appreciation
    2: with the help of or owing to; "thanks to hard work it was a
       great success"

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "thanks":
   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,
   thanksgiving