In the United States, a state is a constituent political entity, of which there are currently 50. Bound together in a political union, each state holds governmental jurisdiction over a separate and defined geographic territory and shares its sovereignty with the federal government. Due to this shared sovereignty, Americans are citizens both of the federal republic and of the state in which they reside. State citizenship and residency are flexible, and no government approval is required to move between states, except for persons restricted by certain types of court orders (e.g., paroled convicts and children of divorced spouses who are sharing custody). Four states use the term commonwealth rather than state in their full official names.
States are divided into counties or county-equivalents, which may be assigned some local governmental authority but are not sovereign. County or county-equivalent structure varies widely by state, and states may also create other local governments. State governments are allocated power by the people (of each respective state) through their individual constitutions. All are grounded in republican principles, and each provides for a government, consisting of three branches, each with separate and independent powers: executive, legislative, and judicial.States possess a number of powers and rights under the United States Constitution. States and their residents are represented in the United States Congress, a bicameral legislature consisting of the Senate and the House of Representatives. Each state is also entitled to select a number of electors (equal to the total number of representatives and senators from that state) to vote in the Electoral College, the body that directly elects the President of the United States. Additionally, each state has the opportunity to ratify constitutional amendments, and, with the consent of Congress, two or more states may enter into interstate compacts with one another.
Historically, the tasks of local law enforcement, public education, public health, regulating intrastate commerce, and local transportation and infrastructure have generally been considered primarily state responsibilities, although all of these now have significant federal funding and regulation as well. Over time, the Constitution has been amended, and the interpretation and application of its provisions have changed. The general tendency has been toward centralization and incorporation, with the federal government playing a much larger role than it once did. There is a continuing debate over states' rights, which concerns the extent and nature of the states' powers and sovereignty in relation to the federal government and the rights of individuals.
The Constitution grants to Congress the authority to admit new states into the Union. Since the establishment of the United States in 1776, the number of states has expanded from the original 13 to 50. Alaska and Hawaii are the most recent states admitted, both in 1959. The Constitution is silent on the question of whether states have the power to secede (withdraw) from the Union. Shortly after the Civil War, the U.S. Supreme Court, in Texas v. White, held that a state cannot unilaterally do so.