Definition from Noah Webster 1828 Dictionary

M’ARQUE,
MARK n. [Fr.] Letters of marque are letters of reprisal;^def a license or extraordinary commission granted by a sovereign of one state to his subjects, to make reprisals at sea on the subjects of another, under pretense of indemnification for injuries received. Marque is said to be from the same root as marches, limits, frontiers, and literally to denote a license to pass the limits of a jurisdiction on land, for the purpose of obtaining satisfaction for theft by seizing the property of the subjects of a foreign nation.