The prisoner's dilemma is a theoretical problem used in game theory, which demonstrates how cooperation, or a lack of it, can affect the outcome of a situation. If two prisoners are separated and each is given the opportunity to betray his accomplice, there are several different outcomes depending on what he decides and what his accomplice does.
Dr. Judson had a logic diagram of the prisoner's dilemma on the blackboard in his office at Maiden's Point. The Doctor recognised it immediately, which enabled him to break the ice with Judson. (DW: The Curse of Fenric)