The Collaborative International Dictionary of English v.0.48:

Guarantee \Guar`an*tee"\, n.; pl. Guarantees. [For guaranty,
   prob. influenced by words like assignee, lessee, etc. See
   Guaranty, and cf. Warrantee.]
   1. In law and common usage: A promise to answer for the
      payment of some debt, or the performance of some duty, in
      case of the failure of another person, who is, in the
      first instance, liable to such payment or performance; an
      engagement which secures or insures another against a
      contingency; a warranty; a security. Same as Guaranty.
      [1913 Webster]

            His interest seemed to be a guarantee for his zeal.
                                                  --Macaulay.
      [1913 Webster]

   2. One who binds himself to see an undertaking of another
      performed; a guarantor. --South.
      [1913 Webster]

   Note: Guarantor is the correct form in this sense.
         [1913 Webster]

   3. (Law) The person to whom a guaranty is made; -- the
      correlative of guarantor.

   Syn: Guarantee, Warranty.

   Usage: A guarantee is an engagement that a certain act will
          be done or not done in future. A warranty is an
          engagement as to the qualities or title of a thing at
          the time of the engagement.
          [1913 Webster]