A U.S. state is a federated state of the United States of America that shares sovereignty with the United States federal government. Since the admission of Hawaii as a state in August 1959, there are fifty U.S. states. Because of the shared sovereignty between a U.S. state and the U.S. federal government, an American is a citizen of both the federal entity and of his or her state of domicile. Kentucky, Massachusetts, Pennsylvania, and Virginia use the official title of Commonwealth rather than State. State citizenship is flexible and no government approval is required to move between states, with the exception of paroled convicts.

The United States Constitution allocates certain powers to the federal government. It also places some limitations on the state governments. State governments are allocated power by the people (of each respective state) through their individual constitutions. By ratifying the United States Constitution, the people transferred certain limited sovereign powers to the federal government from their states. Under the Tenth Amendment, all powers not delegated to the federal government nor prohibited to the states are retained by the states or the people. Historically, the tasks of public safety (in the sense of controlling crime), public education, public health, transportation, and infrastructure have generally been considered primarily state responsibilities, although all of these now have significant federal funding and regulation as well (based largely upon the Commerce Clause, the Taxing and Spending Clause, and the Necessary and Proper Clause of the Constitution).

The United States of America (commonly abbreviated to the United States, the U.S., the USA, America, and the States) is a federal constitutional republic comprising fifty states and a federal district. The country is situated mostly in central North America, where its forty-eight contiguous states and Washington, D.C., the capital district, lie between the Pacific and Atlantic Oceans, bordered by Canada to the north and Mexico to the south. The state of Alaska is in the northwest of the continent, with Canada to the east and Russia to the west, across the Bering Strait. The state of Hawaii is an archipelago in the mid-Pacific. The country also possesses several territories in the Pacific and Caribbean.

At 3.79 million square miles (9.83 million km²) and with over 312 million people, the United States is the third or fourth largest country by total area, and the third largest by both land area and population. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries. The U.S. economy is the world's largest national economy, with an estimated 2011 GDP of $15.1 trillion (22% of nominal global GDP and over 19% of global GDP at purchasing-power parity). Per capita income is the world's sixth-highest.