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]

The Collaborative International Dictionary of English v.0.48:

Guarantee \Guar"an*tee`\, v. t. [imp. & p. p. guaranteed; p,
   pr. & vb. n. Guaranteeing.] [From Guarantee, n.]
   In law and common usage: to undertake or engage for the
   payment of (a debt) or the performance of (a duty) by another
   person; to undertake to secure (a possession, right, claim,
   etc.) to another against a specified contingency, or in all
   events; to give a guarantee concerning; to engage, assure, or
   secure as a thing that may be depended on; to warrant; as, to
   guarantee the execution of a treaty.
   [1913 Webster]

         The United States shall guarantee to every State in
         this Union a republican form of government.
                                                  --Constitution
                                                  of the U. S.
   [1913 Webster]

WordNet (r) 3.0 (2006):

guarantee
    n 1: a written assurance that some product or service will be
         provided or will meet certain specifications [syn:
         guarantee, warrant, warrantee, warranty]
    2: an unconditional commitment that something will happen or
       that something is true; "there is no guarantee that they are
       not lying"
    3: a collateral agreement to answer for the debt of another in
       case that person defaults [syn: guarantee, guaranty]
    v 1: give surety or assume responsibility; "I vouch for the
         quality of my products" [syn: guarantee, vouch]
    2: make certain of; "This nest egg will ensure a nice retirement
       for us"; "Preparation will guarantee success!" [syn:
       guarantee, ensure, insure, assure, secure]
    3: promise to do or accomplish; "guarantee to free the
       prisoners" [syn: undertake, guarantee]
    4: stand behind and guarantee the quality, accuracy, or
       condition of; "The dealer warrants all the cars he sells"; "I
       warrant this information" [syn: guarantee, warrant]

Moby Thesaurus II by Grady Ward, 1.0:

118 Moby Thesaurus words for "guarantee":
   Bible oath, and candle, arm, armor, assert under oath, assurance,
   assure, attest, avouch, avouchment, avow, back, bail, bailsman,
   be sponsor for, bless, bond, bondsman, book, certify, champion,
   cloak, compass about, confirm, copyright, countersecure,
   countersign, cover, cushion, defend, depone, depose, earnest,
   endorse, ensure, extrajudicial oath, faith, fence, fend, godfather,
   godparent, guarantor, guaranty, guard, harbor, haven, indemnity,
   insurance, insure, insurer, ironclad oath, judicial oath, keep,
   keep from harm, kiss the book, loyalty oath, mainpernor,
   make a promise, make certain, make safe, make sure, mortgagor,
   nestle, oath, oath of allegiance, oath of office, obligation,
   official oath, parole, patent, pledge, plight, police, promise,
   protect, register, ride shotgun for, safeguard, screen, secure,
   security, shelter, shield, shroud, sign, sign for,
   solemn declaration, solemn oath, sponsor, stand behind,
   stand up for, stocks and bonds, subscribe to, surety, swear,
   swear by bell, swear the truth, swear to, swear to God,
   swear to goodness, test oath, testify, tie, token, troth,
   undersign, undertake, undertaking, underwrite, underwriter, vouch,
   vouch for, vow, warrant, warrantor, warranty, word,
   word of honor



Bouvier's Law Dictionary, Revised 6th Ed (1856):

GUARANTEE, contracts. He lo whom a guaranty is made.
     2. The guarantee is entitled to receive payment, in the first place,
from the debtor, and, secondly, from the guarantor. He must be careful not
to give time beyond that stipulated in the original agreement, to the
debtor, without the consent of the guarantor; the guarantee should, at the
instance of the guarantor, bring an action against the principal for the
recovery of the debt. 2 Johns. Oh. R. 554; 17 Johns. R. 384; 8 Serg. &
Rawle, 116; 10 Serg. & Rawle, 33; 2 Bro. C. C. 579, 582; 2 Ves. jr. 542. But
the mere omission of the guarantee to sue the principal debtor will not, in
general, discharge the guarantor. 8 Serg. & Rawle, 112; 3 Yeates, R. 157; 6
Binn. R. 292, 300.