Lettres de Cachet (i.e. sealed letters), warrants of imprisonment, issued prior to the Revolution, sealed with the private seal of the king, in contradistinction from lettres patentées, which were sealed by the Great Seal of the kingdom. See Cachet, Lettre de.
Definition taken from The Nuttall Encyclopædia, edited by the Reverend James Wood (1907)
Letters Patent * Leucippus