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.

Piracy is an act of robbery or criminal violence by ship or boat-borne attackers upon another ship or a coastal area, typically with the goal of stealing cargo and other valuable goods. Those who conduct acts of piracy are called pirates, and vessels used for piracy are called pirate ships. The earliest documented instances of piracy were in the 14th century BC, when the Sea Peoples, a group of ocean raiders, attacked the ships of the Aegean and Mediterranean civilisations. Narrow channels which funnel shipping into predictable routes have long created opportunities for piracy, as well as for privateering and commerce raiding.