Privateer

A privateer was a privately-owned ship that was given an official government letter of marque to raid enemy shipping during wartime. The term also applies to sailors on such ships.
Well-known privateers included Francis Drake and Martin Frobisher, both from England.
United States.
During the American Revolution, privateers were used by the United States to raid British shipping.
During the War of 1812, privateers were again used to raid British shipping.
During the American Civil War, the Confederate States issued privateers letters of marque.