letter of marque

noun
1.
license or commission granted by a state to a private citizen to capture and confiscate the merchant ships of another nation.
Also, letters of marque.
Also called letter of marque and reprisal, letters of marque and reprisal.
Origin
1400-50; late Middle English
British Dictionary definitions for letter of marque

letter of marque

noun
1.
a licence granted by a state to a private citizen to arm a ship and seize merchant vessels of another nation
2.
a similar licence issued by a nation allowing a private citizen to seize goods or citizens of another nation
Also called letter of marque and reprisal