a commission issued in Britain by the Lord High Admiral or Commissioners of the Admiralty authorizing the commander of a privately owned ship to cruise in search of enemy merchant vessels. The letter of marque described the ship, her owners and officers, the amount of surety which had been deposited and stressed the necessity of having all prize vessels or goods seized condemned and valued at a Vice Admiralty Court for the payment of 'prize money'.

This was a legal license given by a countries government giving permission to seize enemy goods and treasure on the open sea. In other words, it meant that you could be a pirate without going to jail. Most letters of marque were given only if the ship's captain agreed to share all treasure that was captured with the country that gave them the license.

The papers given a privateer authorizing him to act. The letter specified the period for which it was valid. Often the limits of the Marque were vague, leaving it up to the captain and crew to determine where to go and what they could seize.