Plenary means characterized by being full and complete in every respect. For example, a plenary trial is a full trial of all the issues, factual and legal. Sometimes when a case is heard on appeal, the hearing is limited to only questions of law. However, an appellate judge may order the case to be sent back to the trial court to be heard again from scratch in a plenary trial. in reference to a legislative session, it refers to being attended or meant to be attended by every member or delegate. Plenary authority refers to the complete power of a governing body.
Plenary is also used to denote 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. 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.
For example, in immigration law, Congress, under the Plenary Power Doctrine, has the power to make immigration policy free from judicial review. This doctrine was established at the end of the nineteenth century, when the Supreme Court declared that Congress had "plenary power" to regulate immigration, Indian tribes, and newly acquired territories. The doctrine is based on the concept that immigration is a question of national sovereignty, relating to a nation's right to define its own borders. Courts generally refrain from interfering in immigration matters. To date there have been no successful challenges to federal legislation that refuses admission to classes of non-citizens or removes resident aliens.