A pirate is one who robs or plunders at sea without a commission from a recognised sovereign nation. Pirates usually target other ships, but have also attacked targets on shore. These acts are known as piracy. The  word pirate (peirato) was first used in about 140 BC by the Roman historian Polybius.

“Piracy” is defined under the 1982 United Nations Convention on the Law of theSea to include illegal acts of violence, detention, or depredation for private endscommitted by the crew or passengers of one ship against another ship (oraircraft), or persons or property on board that ship. Piracy per se occurs ininternational waters, outside the jurisdiction of any state, and usually outsidesecurity patrolled or monitored areas.

Because it is often the result of failure or laxity in patrolling sea routes, piracy flourished in times of unrest, or when navies ordinarily protecting commerce were engaged in war. Pirates found their most suitable base of operations in an archipelago that offered shelter together with proximity to trade routes.

In modern times ships, as well as aeroplanes, are also hijacked for political reasons. The perpetrators of these acts could be described as pirates (for instance, the French for plane hijacker is pirate de l'air), but in English are usually termed hijackers or terrorists. An example is the hijacking of the Italian civilian passenger ship, the Achille Lauro.