A prisoner is simply a restricted person. He is someone confined in limited by force of law. He may not leave that space until such a time as a superior authority will permit.
In a gone age, the Portuguese used to confine convicts to jail terms in Africa. The whole continent was seen as jail. And so a judge in Lisbon would say: “The court sentences you to six years in Africa.” They would load you into a ship and freight you to Africa, where you would be let loose.