A pirate is a person who commits a crime while at sea. The crimes could be anything but usually involve robbery or violence. Arr!!
Although people generally associate traditional pirates as those that caused trouble and mayhem in and around the Caribbean in the 17th and 18th Centuries, drinking grog and rum and speaking with some kind of Cornish accent, pirates have existed and still exist wherever there is water, which is just about everywhere. For example, the Vikings were also pirates.