The Collaborative International Dictionary of English v.0.48:

Cachet \Cach"et\, n. [F. fr. cacher to hide.]
   A seal, as of a letter.
   [1913 Webster]

   Lettre de cachet [F.], a sealed letter, especially a letter
      or missive emanating from the sovereign; -- much used in
      France before the Revolution as an arbitrary order of
      imprisonment.
      [1913 Webster] Cachexia

WordNet (r) 3.0 (2006):

cachet
    n 1: an indication of approved or superior status [syn:
         cachet, seal, seal of approval]
    2: a warrant formerly issued by a French king who could warrant
       imprisonment or death in a signed letter under his seal [syn:
       cachet, lettre de cachet]
    3: a seal on a letter

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "cachet":
   aroma, attribute, badge, billhead, book stamp, bookplate, brand,
   broad arrow, cast, character, characteristic, check, colophon,
   configuration, counterfoil, countermark, cut, differentia,
   differential, dignity, distinction, distinctive feature, docket,
   earmark, feature, figure, flavor, government mark,
   government stamp, gust, hallmark, identification, idiocrasy,
   idiosyncrasy, importance, impress, impression, imprint, index,
   individualism, keynote, label, letterhead, lineaments, logo,
   logotype, mannerism, mark, marking, masthead, mold, nature, odor,
   particularity, peculiarity, plate, prestige, price tag, prominence,
   property, quality, quirk, registered trademark, running head,
   running title, savor, seal, shape, sigil, signet, singularity,
   smack, specialty, stamp, sticker, stub, tag, taint, tally, tang,
   taste, ticket, title page, token, trade name, trademark,
   trademark name, trait, trick