Derived from the latin term "plenus" meaning "full", plenary authority refers to the complete power of a governing body. The concept is also used in legal circles 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 collatoral or by a legal claim.

The plenary power of the U.S. Congress, or of other sovereign nations, allow 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 rights of states and of individuals, are held to limit the plenary power of Congress, Chief 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 Ass'n (1981)[1] Judge Robert Bork stated: "the expansion of Congress's commerce, taxing and spending powers has reached a point where it is not possible to state that, as a matter of articulated doctrine, there are any limits left."

Politically, conservatives tend to have more complaints about expanded exercise of plenary power, as progressive platforms more often depend on those very powers. The United States Constitution vests plenary power in Congress and courts have been reluctant to intervene in many cases. Congress' plenary power, by some arguments, competes with the right of citizens to enjoy equal protection under the law. Some contend courts have been reluctant to enforce equal protection out of deference to the plenary power of Congress.