A U.S. state is any one of 50 subnational entities of the United States of America that share sovereignty with the federal government (four states use the official title of commonwealth rather than state). Because of this shared sovereignty, an American is a citizen both of the federal entity and of his or her state of domicile. However, state citizenship is very flexible, and no government approval is required to move between states (with the exception of convicts on parole).

The United States Constitution allocates power between the two levels of government. By ratifying the Constitution, each state transferred certain, but limited, sovereign powers to the federal government. Under the Tenth Amendment, all powers not delegated to the U.S. government nor prohibited to the states are retained by the states, or the people. Historically, the tasks of public education, public health, transportation and other infrastructure have been considered primarily state responsibilities, although they 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 that of the federal government and their power over individuals.