Detention is the process whereby a state or private citizen lawfully holds a person by removing their freedom or liberty at that time. This can be due to (pending) criminal charges preferred against the individual pursuant to a prosecution or to protect a person or property. Being detained does not always result in being taken to a particular area (generally called a detention centre), either for interrogation or as punishment for a crime (see prison). An individual may be detained due a psychiatric disorder, potentially to treat this disorder involuntarily. They may also be detained for to prevent the spread of infectious diseases such as tuberculosis.

The term can also be used in reference to the holding of property for the same reasons. The process of detainment may or may not have been preceded or followed with an arrest.

Detainee is a term used by certain governments and their armed forces to refer to individuals held in custody, such as those it does not classify and treat as either prisoners of war or suspects in criminal cases. It is used to refer to "any person captured or otherwise detained by an armed force." More generally, it means "someone held in custody." The prisoners in Guantánamo Bay are referred to as "detainees".

Article 9 of the Universal Declaration of Human Rights provides that "[n]o one shall be subjected to arbitrary arrest, detention or exile." In wars between nations, treatment of detainees is governed by the provisions of the Fourth Geneva Convention.