A privateer is a private person or vessel which engages in maritime warfare under a commission of war. Since robbery under arms was a common aspect of seaborne trade, until the early 19th century all merchant ships carried arms. A sovereign or delegated authority issued commissions, also referred to as letters of marque, during wartime. The commission empowered the holder to carry on all forms of hostility permissible at sea by the usages of war. This included attacking foreign vessels and taking them as prizes and taking crews prisoner for exchange. Captured ships were subject to condemnation and sale under prize law, with the proceeds divided by percentage between the privateer's sponsors, shipowners, captains and crew. A percentage share usually went to the issuer of the commission.

Thomas Tew, also known as the Rhode Island Pirate, was a 17th-century English privateer-turned-pirate. He embarked on two major pirate voyages and met a bloody death on the second, and he pioneered the route which became known as the Pirate Round. Other infamous pirates in his path included Henry Avery and William Kidd.