constituent political entity of the United States
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.
U.S. state on wikipedia