
Papers issued by a national government during wartime, entitling a privately owned ship to raid enemy commerce, or even attack enemy warships. Early letters of reprisal were issued to merchants to make it legal for them to counter-raid pirates! A ship bearing such letters, and operating within their limits, is a privateer rather than a pirate . . ....
Found on
http://sj.sjgames.com/PirateTalk.html

proof that a pirate/privateer is sponsored by a particular government.
Found on
http://www.encyclo.co.uk/local/22083

Letters of Marque or Letters of Marque and Reprisal were a license or extraordinary commission granted by a sovereign or the supreme power of one state to the citizens of this state to make reprisals at sea on the subjects of another, under pretence of indemnification for injuries received;
Found on
http://www.probertencyclopaedia.com/browse/AL.HTM

commission given to a non-military sea captain to plunder in the name of the Crown (allowed to capture enemy ships and split the money with the Crown)
Found on
https://terrancezepke.com/pirates/pirate-glossary/

Government-issued letters allowing privateers the right to piracy of another ship during wartime
Found on
https://www.piratevoyages.com/pirate-lingo/
No exact match found.