Solitary confinement is a form of imprisonment in which an inmate is isolated from any human contact, often with the exception of members of prison staff, for 22–24 hours a day, with a sentence ranging from days to decades. It is mostly employed as a form of punishment beyond incarceration for a prisoner, usually for violations of prison regulations. However, it is also used as an additional measure of protection for vulnerable inmates. In the case of prisoners at high risk of suicide, it can be used to prevent access to items that could allow the prisoner to self-harm.
Solitary confinement is colloquially referred to in American English as "the hotbox", "the hole", the BOX, the Bing, "lockdown", HDM (Housing Detention for Men), "AdSeg" (administrative segregation), the "SHU" (pronounced "shoe"), an acronym for "Special Housing Unit" or "Security Housing Unit", or "the pound"; in British English as "the block", "The Segregation Unit", or "the cooler". It has also been called prison "'segregation' and 'restrictive housing.'"
Solitary confinement receives severe criticism for having detrimental psychological effects and constituting torture.