Piracy is an act of robbery or criminal violence at sea. The term can include acts committed on land, in the air, or in other major bodies of water or on a shore. It does not normally include crimes committed against persons traveling on the same vessel as the perpetrator (e.g. one passenger stealing from others on the same vessel). The term has been used to refer to raids across land borders by non-state agents.

Piracy is the name of a specific crime under customary international law and also the name of a number of crimes under the municipal law of a number of States. It is distinguished from privateering, which is authorized by national authorities and therefore a legitimate form of war-like activity by non-state actors. Privateering is considered commerce raiding, and was outlawed by the Peace of Westphalia (1648) for signatories to those treaties.

Those who engage in acts of piracy are called pirates. Historically, offenders have usually been apprehended by military personnel and tried by military tribunals.