Habeas corpus ( (listen); from Medieval Latin, lit.'that you have the body'; in law: "[we, a Court, command] that you have the body [of the detainee brought before us]") is a recourse in law through which a person can report an unlawful detention or imprisonment to a court and request that the court order the custodian of the person, usually a prison official, to bring the prisoner to court, to determine whether the detention is lawful.The writ of habeas corpus was described in the eighteenth century by William Blackstone as a "great and efficacious writ in all manner of illegal confinement". It is a summons with the force of a court order; it is addressed to the custodian (a prison official, for example) and demands that a prisoner be brought before the court, and that the custodian present proof of authority, allowing the court to determine whether the custodian has lawful authority to detain the prisoner. If the custodian is acting beyond their authority, then the prisoner must be released. Any prisoner, or another person acting on their behalf, may petition the court, or a judge, for a writ of habeas corpus. One reason for the writ to be sought by a person other than the prisoner is that the detainee might be held incommunicado. Most civil law jurisdictions provide a similar remedy for those unlawfully detained, but this is not always called habeas corpus. For example, in some Spanish-speaking nations, the equivalent remedy for unlawful imprisonment is the amparo de libertad ("protection of freedom").

Habeas corpus has certain limitations. Though a writ of right, it is not a writ of course. It is technically only a procedural remedy; it is a guarantee against any detention that is forbidden by law, but it does not necessarily protect other rights, such as the entitlement to a fair trial. So if an imposition such as internment without trial is permitted by the law, then habeas corpus may not be a useful remedy. In some countries, the writ has been temporarily or permanently suspended under the pretext of a war or state of emergency, for example by Abraham Lincoln during the American Civil War (see Habeas Corpus Suspension Act (1863)).The right to petition for a writ of habeas corpus has nonetheless long been celebrated as the most efficient safeguard of the liberty of the subject. The jurist Albert Venn Dicey wrote that the British Habeas Corpus Acts "declare no principle and define no rights, but they are for practical purposes worth a hundred constitutional articles guaranteeing individual liberty".The writ of habeas corpus is one of what are called the "extraordinary", "common law", or "prerogative writs", which were historically issued by the English courts in the name of the monarch to control inferior courts and public authorities within the kingdom. The most common of the other such prerogative writs are quo warranto, prohibito, mandamus, procedendo, and certiorari. The due process for such petitions is not simply civil or criminal, because they incorporate the presumption of non-authority. The official who is the respondent must prove their authority to do or not do something. Failing this, the court must decide for the petitioner, who may be any person, not just an interested party. This differs from a motion in a civil process in which the movant must have standing, and bears the burden of proof.

The president of the United States (POTUS) is the head of state and head of government of the United States of America. The president directs the executive branch of the federal government and is the commander-in-chief of the United States Armed Forces.

The power of the presidency has grown substantially since the office's establishment in 1789. While presidential power has ebbed and flowed over time, the presidency has played an increasingly strong role in American political life since the beginning of the 20th century, with a notable expansion during the presidency of Franklin D. Roosevelt. In contemporary times, the president is also looked upon as one of the world's most powerful political figures as the leader of the only remaining global superpower. As the leader of the nation with the largest economy by nominal GDP, the president possesses significant domestic and international hard and soft power.

Article II of the Constitution establishes the executive branch of the federal government and vests the executive power in the president. The power includes the execution and enforcement of federal law and the responsibility to appoint federal executive, diplomatic, regulatory, and judicial officers. Based on constitutional provisions empowering the president to appoint and receive ambassadors and conclude treaties with foreign powers, and on subsequent laws enacted by Congress, the modern presidency has primary responsibility for conducting U.S. foreign policy. The role includes responsibility for directing the world's most expensive military, which has the second largest nuclear arsenal.

The president also plays a leading role in federal legislation and domestic policymaking. As part of the system of checks and balances, Article I, Section 7 of the Constitution gives the president the power to sign or veto federal legislation. Since modern presidents are also typically viewed as the leaders of their political parties, major policymaking is significantly shaped by the outcome of presidential elections, with presidents taking an active role in promoting their policy priorities to members of Congress who are often electorally dependent on the president. In recent decades, presidents have also made increasing use of executive orders, agency regulations, and judicial appointments to shape domestic policy.

The president is elected indirectly through the Electoral College to a four-year term, along with the vice president. Under the Twenty-second Amendment, ratified in 1951, no person who has been elected to two presidential terms may be elected to a third. In addition, nine vice presidents have become president by virtue of a president's intra-term death or resignation. In all, 45 individuals have served 46 presidencies spanning 58 full four-year terms.Joe Biden is the 46th and current president of the United States, having assumed office on January 20, 2021.