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A plenary power or plenary authority is the separate identification, definition, and complete vesting of a power or powers or authority in a governing body or individual, to choose to act (or not to act) on a particular subject matter or area. The concept is also used in legal contexts to define complete control in other circumstances, as in plenary authority over public funds, as opposed to limited authority over funds that are encumbered as collateral or by a legal claim. It is derived from the Latin term plenus ("full").
United States 
In United States constitutional law, plenary power is a power that has been granted to a body in absolute terms, with no review of, or limitations upon, the exercise of the power. The assignment of a plenary power to one body divests all other bodies from the right to exercise that power, and where not otherwise entitled; also, the right to substantively review the exercise of that power in a particular instance or in general.
There are very few clear examples of such powers in the United States due to the nature of the Constitution which grants different but at times overlapping roles to the three branches of federal government or indicates the states retain certain roles. For example although United States Congress under Article I, Section 8, Clause 3, (the Commerce Clause) has been said to have "plenary" power over Interstate Commerce this does not always preclude the states from passing laws that affect interstate commerce. When an activity is in interstate commerce and when Congress has not regulated it the states can regulate this activity as long as they do so appropriately. This is known as the "Dormant Commerce Clause." Similarly, this is true between the branches. Congress does appear to have complete and absolute power regarding the declaration of war and peace in Article I Section 8 Clause 11. Yet, the President has control over the Armed Forces as Commander-in-Chief. These powers are in ongoing conflict as seen by the War Powers Resolution of 1973. Under what circumstances can the president act in his capacity as Commander-in-Chief without a declaration of war? To what extent must Congress formally declare war, or are resolutions supporting Executive action sufficient?
Furthermore, while it would appear (as seen in the paragraph below) that Congress has Plenary power to levy taxes via the Federal Government, this is a power that is held concurrently with the states.
The plenary power of the U.S. Congress, or of other sovereign nations, allows them to pass laws, levy taxes, wage wars, and hold in custody those who offend against their laws. While other legal doctrines, such as the powers of states and rights of individuals, are held to limit the plenary power of Congress, then-Associate Justice William Rehnquist said the idea of limited federal powers is "one of the greatest 'fictions' of our federalist system" (Hodel v. Virginia Surface Mining & Reclamation Association, 1981). A striking example can be seen in United States v. Kagama, where the Supreme Court found that Congress had complete authority over all Native American affairs. The idea of unlimited Federal powers is at odds with the reality that the 13 original states existed before the U.S. Constitution, that they each became vested with plenary sovereignty inherited directly from the British monarch and Parliament after the joint 1776 Declaration of Independence, and that they voluntarily delegated some of that plenary sovereignty to the Federal government they created by ratifying the U.S. Constitution, which is therefore a government of limited, enumerated powers. This latter view is validated by the Tenth Amendment and the legal doctrine of the equality of states, especially as it relates to the 37 non-original states.
The Congress may create and charter, through the enactment of statutes, corporate bodies (Federal Corporations) who can be granted (through the Congresses’ plenary power to legislate) derivative (derived from the legislation, as opposed to the Constitution itself) plenary power(s) in areas that are defined by statute; and, which comport with the constitution. The Tennessee Valley Authority (TVA) is such an entity. It was created by the Congress as a Federal Corporation. And by statute, the TVA is given plenary authority over the setting the rates (prices) it will charge customers for the electricity that it generates. The Congress effectively gave the TVA plenary power over its generated electricity rate (price) setting process; by statutorily making TVA's rate (price) settings exempt and immune from legal review, by any process whatsoever be it State, Federal or otherwise. Once the TVA Act itself was ruled constitutional, its rate setting process received its derivative plenary power.
There is a difference in reach of plenary powers. While in the TVA example the Congress may at any time amend or remove TVA's plenary power to set the rates for the electricity it sells; the President’s plenary power to pardon or commute those convicted under the laws of the United States, is beyond the reach of the processes of the Federal Government; and requires the amendment of the US Constitution, making it a truly plenary grant of power.
Presidential pardons 
An example of a plenary power granted to an individual is the power to grant pardons for Federal crimes (not State crimes), which is bestowed upon the President of the United States under Article II, Section 2, of the US Constitution. The President is granted the power to: "...grant Reprieves and Pardons for Offences against the United States, except in Cases of Impeachment."
That is, within the defined zone, (e.g., all offences against the United States, except Impeachment) the President may modify the punishment, up to the eradication of the fact of conviction and punishment, for offenses against the United States, entirely. And once done, the President’s exercise of this power may not be reviewed by any body or through any forum. Nor can this self-executing power (because it is self-executing), once exercised, by a President, be reversed, or "taken back", by either the granting President, or any of his/her successors.
Neither the power to grant pardon nor the power to construct the scope of a pardon (a commutation) is within the reach of any subsequent review or alteration. Furthermore, double jeopardy prohibits any subsequent prosecution for the offenses over which the pardon was granted. Even the President himself may not rescind a pardon that either he or a predecessor President has granted, once such pardon is executed (i.e., once the official instrument is signed by the President and sealed on behalf of the United States).
The President may also (as in the case of President Gerald Ford and the then former President Richard Nixon; as well as President George H.W. Bush and the former Secretary of Defense Casper W. Weinberger); prospectively, proclaim a grant of pardon. That is the President may proclaim the pardon of an individual; a group; a corporation; or any entity, chargeable of offences, under Federal law; prospectively making the subject immune from Federal prosecution, for past present or future criminal acts.
Such a pardon does this by destroying the possibility of a prosecution having a purposeful meaning or result. The rules of judicial procedure make such a future prosecution, or the continuance of an on-going prosecution, moot. And thereby, a motion for dismissal of an on-going prosecution; or, of an initiated future prosecution, is granted by a Court, on the grounds that the prosecution would be of no purpose or effect; and that it would needlessly waste a Court’s time and the resources of an accused, who would only have the charges (for which a pardon had been proclaimed), dismissed anyway.
This new post-Watergate, style of pardoning by proclamation (i.e., the proclaiming of a subject to be un-chargeable for offences against the law, notwithstanding that no offences have as yet been charged) is very controversial. Since it is in fact, more of a seeking to continue to cover-up the true extent of a criminal’s wrongdoing; and, the true scope of a conspiracy. It was used in the case of Nixon’s pardon by Gerald Ford, not the forgiving of the perpetration of offences against the law; whose nature, scope and consequences had been fully revealed, by a dispassionate judicial examination. It was a continuation of the cover-up of crimes against the United States, by Nixon and other co-conspirators. Those co-conspirators were forced to bear the full weight of the conspiracy; since Nixon’s role in the shared work and leadership of the conspiracy to obstruct justice; suborned perjury; and bribe witnesses; was fully known, but it was never judicially examined or revealed.
Immigration law 
Federal policy on immigration has been founded on the plenary power doctrine, which holds that the political branches — the legislative and the executive — have sole power to regulate all aspects of immigration as a basic attribute of sovereignty. Historically, the U.S. Supreme Court has taken a hands-off approach when asked to review the political branches' immigration decisions and policy-making, giving Congress and the executive branch the ability to regulate immigration largely without judicial intervention. Some analysts suggest that there is a movement to "erode" political-branch control over immigration in favor of a judge-administered system and that the results have created national security concerns, such as the release of criminal immigrants into U.S. society. The U.S. Supreme Court case Zadvydas v. Davis is cited as an example of the U.S. Supreme Court not following plenary power precedent.
See also 
- Jon Feere, Plenary Power: Should Judges Control U.S. Immigration Policy? Center for Immigration Studies, February 2009.
- Gabriel J. Chin, Is There a Plenary Power Doctrine? A Tentative Apology and Prediction for Our Strange but Unexceptional Constitutional Immigration Law Georgetown Immigration Law Review, Vol. 14, No. 2, p. 257, 2000.