A prison Age is generally any Age from which no return is possible. These Ages were typically uninhabited, except by the person or people imprisoned in them.
The D'ni used prison Ages to incarcerate criminals that they felt needed to be kept separate from the rest of society. These Ages typically still had Linking Books which led back to D'ni, but they were heavily secured and guarded by the Guild of Maintainers. Since the fall of D'ni, the term "prison Age" has also been applied to any Age without any Linking Books in it. Technically, this means that every Age is a prison Age until the first Linking Book is taken there. However, the term is typically only applied when an Age has been intentionally left without a Linking Book back. Ages such as Haven, Spire, and Taghira were designed from the beginning to serve as prison Ages. Others, like Riven, were made into prison Ages at a later date by burning all of the Linking Books on the island.
The term "prison Age" should also not be confused with the apocryphal form of Book known as a Trap Book.