1 A ship used in time of war, generally in the days of sailing navies but occasionally in the early days of steam propulsion, when it was required to negotiate with an enemy. The sign of a ship used as a cartel was the flying of a white flag, a sign that was universally recognized and gave immunity from gunfire when approaching an enemy. Ships' boats, also flying a white flag, were sometimes used as cartels before and after battle in cases where the passing of some special message between belligerents was required.
2 An agreement between two belligerent nations for the exchange of prisoners of war while the war was being waged. See also warfare at sea.