A prisoner was an individual who was captured and held in confinement by a third party.
During the Clone Wars, both the Galactic Republic and the Confederacy of Independent Systems took prisoners. Early in the Clone Wars, the Confederacy had clone troopers taken to Regent Queb's compound. They also had Jedi taken to the Citadel. The Republic kept their prisoners at the Naval Intelligence headquarters on Coruscant.
During the Galactic Civil War, Princess Leia Organa was held prisoner by Darth Vader before she was rescued by Luke Skywalker, Han Solo, and Chewbacca assigned the designation of "Prisoner 2187."