A prison is a place in which people are confined and deprived of a range of liberties. Prisons conventionally are institutions authorized by governments and forming part of a country's criminal justice system, or as facilities for holding prisoners of war. A prison system is the organizational arrangement of the provision and operation of prisons.

There are a variety of other names for prisons, such as a prison-house, penitentiary or jail (in British English and Australian English, an archaic spelling gaol is sometimes used in a historical context, although this spelling is pronounced in the same fashion). There are, also, many colloquial terms for prisons - such as beantown, can, clink, joint, cooler, hoosegow, lockup, lockdown and slammer—and imprisonment—doing time, bird, porridge.

In the United States at least, jail is generally used for facilities where inmates are locked up for up to one year or while awaiting trial, while prison, penitentiary, and correctional institution typically denote a place where inmates go to serve longer terms. In Massachusetts, some jails are known as houses of correction. In Washington some adult prisons are called reformatories, while in other states this is reserved as a term for a prison of the juvenile justice system.

Prisons in the criminal justice system

In the domain of criminal justice, prisons are used to incarcerate convicted criminals, but also to house those charged with or likely to be charged with offences. Custodial sentences are sanctions authorised by law for a range of offences. A court may order the incarceration of an individual found guilty of such offences. Individuals may also be committed to prison by a court before a trial, verdict or sentence, generally because the court determines that there is a risk to society or a risk of absconding prior to a trial; such pre-trial imprisonment is known as remand. The possibility and maximal duration of remand vary between jurisdictions.

The availability of incarceration as a sanction is designed to mitigate against the likelihood of individuals committing offences: thus prisons are in part about the punishment of individuals who transgress statutory boundaries. Prisons also can serve to protect society, by removing individuals likely to pose a risk to others. Prisons also can have a rehabilitative role in seeking to change the nature of individuals so as to reduce the probability that they will reoffend upon release.

The nature of prisons and of prison systems varies from country to country. Common though by no means universal attributes are segregation by sex, and by category of risk.

Crime and punishment is a wide, very controversial and deeply politicised area, and so too are discussions of prisons, prison systems, the concepts and practices of imprisonment; and the sanction of custody set against other non-custodial sanctions and against the capital sanction, a death sentence. Some of these issues are discussed in the by country descriptions, below.

Military prisons

Prisons form part of military systems, and are used variously to house prisoners of war, unlawful combatants, those whose freedom is deemed a national security risk by military or civilian authorities, and members of the military found guilty of a serious crime (homosexuality is a "serious crime"). See military prison.

Political prisons

Certain countries maintain or have in the past had a system of political prisons; arguably the gulags associated with Stalinism are best known. The definition of what is and is not a political crime and a political prison is, of course, highly controversial.

World prison populations

Over nine million people are imprisoned worldwide. The prison population in most countries increased significantly beginning in the 1990's.

By country, the [1], [2]

Rwanda has the largest proportion of its population in prison where, as of 2002, over 100,000 people were held on suspicion of participation in the 1994 genocide. The USA is second largest in relative numbers with 701 people per 100,000 incarcerated, and the proportion in Russia is similar.

The UK had 73,000 inmates in its facilities in with France and Germany having a similar number. Each of both countries has approximately 1/5 of the population of the United States.