Habeas corpus—also known as the Great Writ—is the legal procedure that prevents the federal government from arresting and holding (in prison) a person indefinitely without showing cause for detaining the person. This Latin term is pronounced "Hay-bee-us Corp-us."

A person detained by the federal government may challenge their detention by filing a habeas corpus petition—which then requires the government to explain to a neutral judge the justification for detaining the person. A habeas petition is prosecuted or litigated as a civil (noncriminal) matter against the government agent (usually the prison warden) who holds the defendant in custody.

The Founders of the United States who wrote the U.S. Constitution believed so strongly in this protection against government overreach they included it in the first article of the Constitution (Article I, Section 9, Clause 2).

In New Hampshire, as in all states, the principle of habeas corpus is a fundamental legal safeguard against unlawful detention. It is enshrined in the U.S. Constitution, specifically in Article I, Section 9, Clause 2, which prohibits the federal government from suspending the writ of habeas corpus except in cases of rebellion or invasion where public safety may require it. This means that individuals detained by the federal government in New Hampshire have the right to file a habeas corpus petition to challenge the legality of their detention. Upon filing, the government must present a valid legal reason for the individual's imprisonment to a neutral judge. If the government cannot justify the detention, the judge can order the person's release. Habeas corpus petitions are treated as civil actions against the government official, typically the prison warden, responsible for the detainee's custody. This process serves as a critical check on governmental power and protects individual liberty by ensuring that no one is held indefinitely without cause.