Solitary confinement is a special form of imprisonment in which a prisoner is isolated from any human contact, though often with the exception of members of prison staff. It is sometimes employed as a form of punishment beyond incarceration for a prisoner and has been cited as an additional measure of protection from the criminal or is given for violations of prison regulations. It is also used as a form of protective custody and to implement a suicide watch.

Solitary confinement is colloquially referred to in American English as the 'hotbox', the 'hole', 'lockdown', 'AdSeg' (Administrative Segregation), the 'SHU' (pronounced 'shoe')—an acronym for security housing unit, or the 'pound'; and in British English as the 'block' or the 'cooler'. In Canada they are known as a Special Handling Unit.

Issues of mental health and insanity due to solitary confinement and other extreme forms of isolation date back to the 19th century and have been broadly researched by a variety of academics and scholars. Historically, Quakers and Calvinists could be seen at either end of the capital punishment argument, the former being against and the latter being strong advocates for it. In 1818, New York reformer, Thomas Eddy, lobbied for inmate labor and solitary confinement in place of other forms of punishment such as hanging. Shortly after, New York decided to include solitary confinement and inmate labor into their penal system. Some of these early prison experiments indicated that prisoners may prefer the lash over solitary, because it did not induce permanent damage and would not incite madness like solitary confinement arguably could. Mental instability has been linked to solitary confinement since as far back as the 1860’s. Prison records from the Denmark institute during 1870-1920 illustrate that staff noticed inmates were exhibiting signs of mental illnesses while in isolation, revealing that this persistent problem has been around for decades.