1. A reformatory prison; a house of correction: A penitentiary is intended for the penal and reformatory treatment of criminals.

Trina was sentenced to forty years in a federal penitentiary for the crime that she committed.
2. 2. In the U.S., the place of punishment in which convicts sentenced to confinement and hard labor are confined by the authority of the law: There are a number of penitentiaries located in the United States including the United States Penitentiary in Pennsylvania, the United States Penitentiary in California, and the United States Penitentiary in Virginia.