Source : Webster's Revised Unabridged Dictionary (1913)

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

   {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.

Source : WordNet®

cachet
     n 1: an indication of approved or superior status [syn: {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: {lettre de cachet}]
     3: a seal on a letter