A prison ship is a starship that acts as a prison. Most times, such vessels were used to transport criminals from one prison or detention facility to another to serve out their sentence.
Prison ships operated by the Nygeans were used in such a fashion. (VOY: "Repentance")
Jonathan Archer and his first officer Charles Tucker III were once prisoners on an Enolian transport serving as a prison ship. (ENT: "Canamar")
Deanna Troi, while portraying Major Rakal on the IRW Khazara, claimed that she had tasted better Viinerine on prison ships when told by Commander Toreth that she should have tried it. (TNG: "Face of the Enemy")