letters of marque (or mart) Extraordinary commissions issued, either in time of war peace, by the Lords of the Admiralty, or the vice-admirals of a distant province, to the commanders of merchant ships, authorising reprisals for reparation of the damages sustained by them through enemies at sea. They were either special, to make reparation to individuals, or general, when issued by the government of one State against all the subjects of another. Letters of countermarque were issued as a reprisal for the issue of letters of marque.