A writ of habeas corpus, from the Latin meaning roughly “you should deliver the body,” is a directive from a judge instructing a person who is detaining another to bring the detainee to a specified place at a specified time for a specified purpose. That is usually done as a protection for the detainee and to avoid illegal incarceration. Such a hearing does not address the question of guilt or innocence, but simply determines if due process has been ensured.
Habeas corpus has its roots in English common law, and was specifically mentioned in the U.S. Constitution (text) (Article 1, Section 9) along with the conditions under which it might be suspended. Most state constitutions also have a provision guaranteeing habeas corpus.
While considering how to construct a Bill of Rights in 1788, Thomas Jefferson wrote to James Madison and as one of the points, considered the suspension of the writ of habeas corpus:
Why suspend the habeas corpus in insurrections and rebellions? The parties who may be arrested, may be charged instantly with a well defined crime; of course, the judge will remand them. If the public safety requires that the government should have a man imprisoned on less probably testimony, in those than in other emergencies, let him be taken and tried, retaken and retried, while the necessity continues, only giving him redress against the government, for damages. Examine the history of England. See how few of the cases of the suspension of the habeas corpus law, have been worthy of that suspension.The most notable suspension of habeas corpus occurred in 1861 when Abraham Lincoln issued a proclamation suspending the right in certain parts of Maryland. He was deeply concerned about keeping Maryland, a slave state located immediately north of Washington, D.C., in the Union. A challenge was mounted to this action in the Merryman Case (1861).