The Convention Against Torture defines torture as “any act by which severe pain or suffering, whether physical or mental, is intentionally inflicted on a person for such purposes as obtaining from him or a third person information or a confession, punishing him for an act he or a third person has committed or is suspected of having committed, or intimidating or coercing him or a third person, or for any reason based on discrimination of any kind, when such pain or suffering is inflicted by or at the instigation of or with the consent or acquiescence of a public official or other person acting in an official capacity.”
Under international law, torture is governed by Article 5 of the Universal Declaration of Human Rights and by Article 7 of the International Covenant on Civil and Political Rights. The Universal Declaration of Human Rights, states, “No one shall be subjected to torture or to cruel, inhuman or degrading treatment or punishment.”
One of the most common punishments for infraction of rules in prison is solitary confinement. At least 32 states operate prisons designed for solitary confinement of all inmates (entirely or in dedicated wings). Prisoners are in cells for 23 hours with an hour daily for exercise in an external cage.
The effects of sensory deprivation following even short periods of isolation have been well documented since the 19th century. Even in the absence of physical brutality or a lack of hygiene, isolation can and does result in emotional damage, a decline in mental function, hallucinations and delusions. These consequences are often longstanding, sometimes permanent, and can occur after very short periods of isolation.
Oscar Lopez Rivera, a Puerto Rican political prisoner still incarcerated in the US after 30 years, has spent at least 12 years in complete isolation.
This punishment is considered torture under international law.
For detailed information on solitary confinement and its effects, one useful text is the Sourcebook on Solitary Confinement by Sharon Shalev (2008).