Latin for "that you have the body." A writ of habeas corpus is used to bring a prisoner or other detainee (e.g. institutionalized mental patient) before the court to determine if the person's imprisonment or detention is lawful. In the US system, federal courts can use the writ of habeas corpus to determine if a state's detention of a prisoner is valid. A habeas petition proceeds as a civil action against the State agent (usually a warden) who holds the defendant in custody. It can also be used to examine any extradition processes used, amount of bail, and the jurisdiction of the court.
See, e.g. Knowles v. Mirzayance 556 U.S.___(2009), Felker v. Turpin 518 US 1051 (1996) and McCleskey v. Zant 499 US 467 (1991).