Letter of marque
A letter of marque is a letter from one nation or crown giving permission to raid ships of other nations, generally as long as a percentage of the spoils were paid to that nation. Letters of Marque are particularly associated with Pirates.
Permission to capture foreign ships was often implicitly given - everyone knew it was fine to do so, and by late period times it was accepted naval practise. Many of the earlier medieval letters exist to grant the ship's owner or captain permission to restock at certain suppliers, with a glancing mention that they happen to be pirates. Or a letter might confirm the distribution of shares of spoils of a raid (between lord or government and crew) or describe a ship's duties with mention of sacking foreign ships as a normal part of ship life.
- Example letters of Marque (dead link)