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 York, as in all states, habeas corpus is a fundamental legal procedure that protects individuals from being detained indefinitely without just cause. It serves as a check against unlawful imprisonment by the federal government. When a person is detained by federal authorities in New York, they have the right to file a habeas corpus petition. This petition compels the government to present a valid reason for the individual's detention before a neutral judge. The process is civil in nature and is directed against the government official, typically the prison warden, who is holding the individual. The importance of habeas corpus is underscored by its inclusion in the U.S. Constitution; specifically, Article I, Section 9, Clause 2, which reflects the Founders' intent to safeguard against government overreach and protect personal liberty.