The Collaborative International Dictionary of English v.0.48:

Indorsement \In*dorse"ment\, n. [From Indorse; cf.
   Endorsement.] [Written also endorsement.]
   [1913 Webster]
   1. The act of writing on the back of a note, bill, or other
      written instrument.
      [1913 Webster]

   2. That which is written on the back of a note, bill, or
      other paper, as a name, an order for, or a receipt of,
      payment, or the return of an officer, etc.; a writing,
      usually upon the back, but sometimes on the face, of a
      negotiable instrument, by which the property therein is
      assigned and transferred. --Story. Byles. Burrill.
      [1913 Webster]

   3. Sanction, support, or approval; as, the indorsement of a
      rumor, an opinion, a course, conduct.
      [1913 Webster]

   Blank indorsement. See under Blank. Indorser

The Collaborative International Dictionary of English v.0.48:

Endorsement \En*dorse"ment\, n. [Cf. F. endossement.]
   Same as Indorsement.
   [1913 Webster]

WordNet (r) 3.0 (2006):

endorsement
    n 1: a promotional statement (as found on the dust jackets of
         books); "the author got all his friends to write blurbs for
         his book" [syn: endorsement, indorsement, blurb]
    2: a speech seconding a motion; "do I hear a second?" [syn:
       second, secondment, endorsement, indorsement]
    3: formal and explicit approval; "a Democrat usually gets the
       union's endorsement" [syn: sanction, countenance,
       endorsement, indorsement, warrant, imprimatur]
    4: a signature that validates something; "the cashier would not
       cash the check without an endorsement" [syn: endorsement,
       indorsement]
    5: the act of endorsing; "a star athlete can make a lot of money
       from endorsements" [syn: endorsement, indorsement]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "endorsement":
   John Hancock, OK, X, acceptance, accord, acquiescence, adherence,
   admiration, affirmance, affirmation, affirmative,
   affirmative voice, agreement, approbation, approval, assent,
   authentication, authorization, autograph, aye, backing, blessing,
   certification, christcross, cipher, compliance, confirmation,
   connivance, consent, countenance, countermark, countersign,
   countersignature, counterstamp, cross, device, eagerness, esteem,
   favor, favorable vote, fiat, go-ahead, green light, hand,
   imprimatur, indorsement, initials, mark, mark of signature,
   monogram, nod, notarization, okay, permission, promptitude,
   promptness, ratification, readiness, respect, rubber stamp,
   sanction, seal, seal of approval, sigil, sign manual, signature,
   signet, stamp, stamp of approval, submission, subscription,
   support, the nod, ungrudgingness, unloathness, unreluctance,
   validation, visa, vise, voice, vote, warrant, willingness, yea,
   yea vote