A prison, also known as a jail, gaol, penitentiary, detention center, correction center, correctional facility, or remand center, is a facility where people are imprisoned under the authority of the state, usually as punishment for various crimes. They may also be used to house those awaiting trial (pre-trial detention). Prisons serve two primary functions within the criminal-justice system: holding people charged with crimes while they await trial, and confining those who have pleaded guilty or been convicted to serve out their sentences.

Prisons can also be used as a tool for political repression by authoritarian regimes who detain perceived opponents for political crimes, often without a fair trial or due process; this use is illegal under most forms of international law governing fair administration of justice. In times of war, belligerents or neutral countries may detain prisoners of war or detainees in military prisons or in prisoner-of-war camps. At any time, states may imprison civilians – sometimes large groups of civilians – in internment camps.

The terminology used to describe or distinguish between prisons and other correctional facilities can vary between nations and jurisdictions.

In Australia, the words "gaol", "jail" and "prison" are commonly used. The spelling "gaol" was in official use in the past, and many historical gaols are now tourist attractions, such as the Maitland Gaol. Officially, the term "correctional centre" is used for almost all prisons in New South Wales and Queensland, while other states and territories use a variety of names. "Prison" is officially used for some facilities in South Australia, Victoria and Western Australia. Youth prisons in Australia are referred to as "youth correctional facilities" or "youth detention centres" among other names, depending on the jurisdiction.

In Canada, while the terms "jail" and "prison" are commonly used in speech, officially named facilities use "facility", "correctional centre", "penitentiary", or "institution". A number of facilities retain their historical designation as a "jail".

In New Zealand, the terms "jail" and "prison" are commonly used, although the term "correctional facility", among others, are in official usage.

In Papua New Guinea, "prison" is officially used, although "jail" is widely understood and more common in usage.

The official modern term is "prison" (e.g. HM Prison Barlinnie). The spelling "gaol" is obsolete in modern speech but is still found in older texts, and in historical and legal contexts.