Habeas corpus is the name of a legal action, or writ, through which a person can seek relief from unlawful detention of himself or another person. The writ of habeas corpus has historically been an important instrument for the safeguarding of individual freedom against arbitrary state action. Also known as "The Great Writ," a writ of habeas corpus ad subjiciendum is a summons with the force of a court order addressed to the custodian (such as a prison official) demanding that a prisoner be brought before the court, together with proof of authority, allowing the court to determine whether that custodian has lawful authority to hold that person, or, if not, the person should be released from custody. The prisoner, or another person on their behalf (for example, where the prisoner is being held incommunicado), may petition the court or an individual judge for a writ of habeas corpus.