The Collaborative International Dictionary of English v.0.48:

Cartel \Car*tel"\, n. [F., fr. LL. cartellus a little paper,
   dim. fr. L. charta. See 1st Card.]
   [1913 Webster]
   1. (Mil.) An agreement between belligerents for the exchange
      of prisoners. --Wilhelm.
      [1913 Webster]

   2. A letter of defiance or challenge; a challenge to single
      combat. [Obs.]
      [1913 Webster]

            He is cowed at the very idea of a cartel., --Sir W.
                                                  Scott.
      [1913 Webster]

   Cartel, or Cartel ship, a ship employed in the exchange
      of prisoners, or in carrying propositions to an enemy; a
      ship beating a flag of truce and privileged from capture.
      [1913 Webster]