A plenary power or plenary authority is a complete and absolute power to take action on a particular issue, with no limitations, e.g. the granting of federal pardons by the President. It is derived from the Latin term plenus ("full").
In United States constitutional law, plenary power is a power that has been granted to a body, or person, in absolute terms, with no review of, or limitations upon, the exercise of that power. The assignment of a plenary power to one body divests all other bodies from the right to exercise that power, where not otherwise entitled. Plenary powers are not subject to judicial review 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 and to the states. For example, although the 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 in some way. When an activity is legally classified as interstate commerce, historically the states can regulate this type of activity as long as they do so within the bounds of their Constitutional authority. 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.
Another example of the ongoing debate over plenary powers in the US Constitution is the controversy surrounding the Spending Clause (Article I, Section 8, Clause 1). This clause states that the Congress is allowed to "lay and collect Taxes, Duties, Imposts and Excises, to pay the Debts and provide for the common Defence [sic] and general Welfare of the United States". How far this clause goes, and what it actually means in practice, has been hotly debated since the ratification of the Constitution.
While other Constitutional doctrines, such as the unenumerated powers of states and the rights of individuals, are widely held (both historically and currently) as limiting the plenary power of Congress, then-Associate Justice William Rehnquist reflected that "one of the greatest 'fictions' of our federal system is that the Congress exercises only those powers delegated to it, while the remainder are reserved to the States or to the people. The manner in which this Court has construed the Commerce Clause amply illustrates the extent of this fiction. Although it is clear that the people, through the States, delegated authority to Congress to "regulate Commerce. . . among the several States," (Commerce Clause), one could easily get the sense from this Court's opinions that the federal system exists only at the sufferance of Congress." (Hodel v. Virginia Surface Mining & Reclamation Association, 1981).