The Collaborative International Dictionary of English v.0.48:

Earnest \Ear"nest\, n. [Prob. corrupted fr. F. arrhes, L. arra,
   arrha, arrhabo, Gr. 'arrabw`n, of Semitic origin, cf. Heb.
   [=e]r[=a]v[=o]n; or perh. fr. W. ernes, akin to Gael. earlas,
   perh. fr. L. arra. Cf. Arles, Earles penny.]
   1. Something given, or a part paid beforehand, as a pledge;
      pledge; handsel; a token of what is to come.
      [1913 Webster]

            Who hath also sealed us, and given the earnest of
            the Spirit in our hearts.             --2 Cor. i.
                                                  22.
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            And from his coffers
            Received the golden earnest of our death. --Shak.
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   2. (Law) Something of value given by the buyer to the seller,
      by way of token or pledge, to bind the bargain and prove
      the sale. --Kent. Ayliffe. Benjamin.
      [1913 Webster]

   Earnest money (Law), money paid as earnest, to bind a
      bargain or to ratify and prove a sale.

   Syn: Earnest, Pledge.

   Usage: These words are here compared as used in their
          figurative sense. Earnest is not so strong as pledge.
          An earnest, like first fruits, gives assurance, or at
          least a high probability, that more is coming of the
          same kind; a pledge, like money deposited, affords
          security and ground of reliance for the future.
          Washington gave earnest of his talent as commander by
          saving his troops after Braddock's defeat; his
          fortitude and that of his soldiers during the winter
          at Valley Forge might rightly be considered a pledge
          of their ultimate triumph.
          [1913 Webster]

WordNet (r) 3.0 (2006):

earnest money
    n 1: money given by a buyer to a seller to bind a contract [syn:
         earnest money, arles]