The United States of America (commonly referred to as the United States, the U.S., the USA, or America) is a federal The structure of federal governments vary from institution to institution based on a broad definition of a basic federal political system, there are two or more levels of government that exist within an established territory and govern through common institutions with overlapping or shared powers as prescribed by a constitution constitutional republic A constitutional republic is a state where the head of state and other officials are elected as representatives of the people, and must govern according to existing constitutional law that limits the government's power over citizens comprising fifty states A U.S. state is any one of 50 federated states of the United States of America that share sovereignty with the federal government . 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 and a federal district Federal districts are a type of administrative division of a federation, under the direct control of the federal government. The country is situated mostly in central North America North America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific Ocean; South America lies to the southeast. North, where its forty-eight contiguous states The term contiguous or continental United States refers to the 48 U.S. states located on the North American continent south of the U.S. border with Canada, plus the District of Columbia. The term excludes the states of Alaska and Hawaii, and all off-shore U.S. territories and possessions, such as Puerto Rico and Washington, D.C. Washington, D.C. , formally the District of Columbia and commonly referred to as Washington, the District, or simply D.C., is the capital of the United States, founded on July 16, 1790. The City of Washington was orig