A privateer is a private warship that attacks and plunders the vessels of enemy nations. The most fundamental difference between privateering and piracy is that a privateer is authorized by Letters of Marque issued by its country's government. Privateers go out without insignia (so for which country they work cannot be proved) and attack the ships of rival or enemy nations. The most famous early privateers were those permitted by Elizabeth I of England to harass the Spanish Armada and disrupt Spanish shipment of precious metals from the Americas. Sir Francis Drake was one of the most celebrated English privateers of the 16th century.
Privateering was a common part of Naval warfare from the 16th to early 19th century. The issuance of Letters of Marque and Reprisal was formally abandoned by the many nations who agreed to the Declaration of Paris in 1856. The US did not agree to the declaration, and as such can still authorize privateering. The US last issued a Letter of Marque to the Airship Resolute in 1941.