Piracy is an act of robbery
Robbery is the crime of taking or attempting to take something of value by force or threat of force or by putting the victim in fear. At common law, robbery is defined as taking the property of another, with the intent to permanently deprive the person of that property, by means of force or fear....

 or criminal violence
Crime is the breach of rules or laws for which some governing authority can ultimately prescribe a conviction...

 at sea. The term can include acts committed on land, in the air, or in other major bodies of water or on a shore
A shore or shoreline is the fringe of land at the edge of a large body of water, such as an ocean, sea, or lake. In Physical Oceanography a shore is the wider fringe that is geologically modified by the action of the body of water past and present, while the beach is at the edge of the 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
Customary international law
Customary international law are those aspects of international law that derive from custom. Along with general principles of law and treaties, custom is considered by the International Court of Justice, jurists, the United Nations, and its member states to be among the primary sources of...

 and also the name of a number of crimes under the municipal law of a number of States.