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®

lettre de cachet
     n : a warrant formerly issued by a French king who could warrant
         imprisonment or death in a signed letter under his seal
         [syn: {cachet}]