also letter of marque
Etymology: Middle English letters of marc, probably part translation of Medieval Latin litterae de marqua
: written authority granted to a private person by a government to seize the subjects of a foreign state or their goods by way of retaliation for injuries ; specifically : a license or extraordinary commission granted by a government to a private person to fit out an armed vessel to cruise as a privateer or corsair at sea and plunder the enemy — called also letters of marque and reprisal