Prison, a public institution where criminals and other offenders are held in confinement as a form of punishment. A prison is a penal institution (from the Latin for “punishment"). Prisons in the United States are also called penitentiaries (places where wrongdoers may repent), correctional institutions, and correctional facilities. Municipal and county prisons are called jails. A jail serves also as a place of detention for persons awaiting trial or execution of sentence, or being held in protective custody. When a jail is a place of detention rather than a penal institution, it may be referred to as a lockup.

Prisons in which adolescents or young adults are confined are often called reformatories. Young offenders are also sent to penal institutions called industrial schools or training schools. Municipal and county penal institutions may be called houses of correction or workhouses.

The traditional prison is designed for close confinement, or maximum security. Many prisoners, however, do not need close confinement, and medium- or minimum-security institutions such as farms and labor camps are found in connection with various prison systems. There are also special institutions for criminals who are mentally ill, addicted to drugs, or who for other reasons need separate confinement.

Prison buildings in the United States reflect the constantly changing attitudes in the field of penology (treatment of criminals). The 19th-century style of penitentiary, a massive structure housing hundreds or thousands of prisoners in cells (small single or double compartments), is still prevalent. Modern prisons tend toward smaller units, with emphasis on cheerful surroundings and exercise and recreation facilities. County and municipal penal quarters are often antiquated and inadequate.