Letter of Marque

A Letter of Marque is a special license that was given to some ship captains which allowed them to commit piracy on an offending country without fear of being booked with piracy.
This was usually a deal made between a country and a ship's captain that required that the captain share all prizes captured with the country that gave him the Letter of Marque.  Most countries gave a Letter of Marque to a captain in order to get payment for goods stolen by another country during acts of piracy.

Navy War History

Marque and Reprisal

Privateer