A penitentiary is a prison or correctional facility that houses people who have been convicted of a crime. The origin of the word "penitentiary" means "to repent" or give penitence for a transgression. While the modern penitentiary continues to house inmates as a form of punishment, its historical counterpart was based on harsher theories of incarceration than those most common in the early 21st century. The modern penitentiary often focuses less on exacting grueling punishment and more on rehabilitation, providing its inmates with treatment and education along with their incarceration when possible.

There are a number of competing interests and goals with regard to sentencing that help define the penitentiary. One major goal is satisfying the need for retribution so citizens do not take punishment into their own hands. Another goal is deterrence — keeping order in society — and providing a disincentive for inmates to reoffend. The goal of rehabilitation is a more modern notion and is the force behind many programs that assist inmates with mental health issues, substance abuse problems and education.

A penitentiary is a complex building, with sections — sometimes even entirely separate buildings — to house female inmates apart from male inmates. Within those sections or buildings, there also are areas to house inmates who haven't yet been tried separate from those who have been tried and are serving their sentence. Still further segregation may separate the worst of the convicted offenders from the general prison population. There also are areas designed for solitary confinement, and areas with cells that are under constant watch; these are often used for inmates believed by prison officials to be suicidal.

Penitentiaries are designed with varying levels of security to house inmates according to the severity of their crime and their perceived potential for violence and escape attempts. These levels of security may be boosted outside with architectural and geographic features such as earthen berms, thick walls, tall fencing, and guard towers. Inside security features can include floor-to-ceiling gates, security cameras, and other design elements that discourage both violence within the building and escape attempts.

Modern penitentiaries frequently include a medical clinic so inmates can receive medical and dental care. They also may contain a chapel or other area that can be used for inmates who want to participate in religious observances. Educational facilities and libraries also are common, and they are utilized in the reform portion of an inmate's sentence to help him get an education — possibly studying to earn a high school equivalency diploma or learn a trade. Such facilities also may be used for therapy sessions for inmates trying to overcome an addiction or deal with a mental health issue.

The penitentiary is only part of the correctional system in the early 21st century. The larger system includes facilities of lesser custody, such as work-release programs and halfway houses, where the prisoner works during the day and returns to a correctional facility to sleep. Probation and parole provide supervision and aftercare for the offender after release from jail or a penitentiary. Other alternatives to a penitentiary include working at unpaid community service, payment of fines, and completion of court-ordered classes or therapy.