The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territories, nine Minor Outlying Islands, and 326 Indian reservations. The United States is the world's third-largest country by both land and total area. It shares land borders with Canada to its north and with Mexico to its south and has maritime borders with the Bahamas, Cuba, Russia, and other nations. With a population of over 333 million, it is the most populous country in the Americas and the third most populous in the world. The national capital of the United States is Washington, D.C. and its most populous city and principal financial center is New York City.

Paleo-Americans migrated from Siberia to the North American mainland at least 12,000 years ago, and are the ancestors of modern Native Americans. The native population declined after European arrival in the 16th century, primarily from diseases such as smallpox and measles, becoming increasingly displaced. Great Britain's Thirteen Colonies, in what is now the eastern U.S., quarreled with the British Crown over taxation and political representation, leading to the American Revolution (1765–1791). After the Revolution, the United States gained independence, the first nation-state founded on Enlightenment principles of liberal democracy. In the late 18th century, the U.S. began expanding across North America. By 1848, it spanned the continent from east to west. Profound sectional division surrounding slavery in the Southern United States led to the secession of the Confederate States of America, which fought the remaining states of the Union during the American Civil War (1861–1865). With the Union's victory and preservation, slavery was abolished nationally by the Thirteenth Amendment.

By 1900, the United States had established itself as a world power, becoming the world's largest economy. After Japan's attack on Pearl Harbor in 1941, the U.S. entered World War II on the Allied side. The aftermath of the war left the United States and the Soviet Union as the world's two superpowers and led to the Cold War. During the Cold War, both countries engaged in a struggle for ideological dominance but avoided direct military conflict. They also competed in the Space Race, which culminated in the 1969 landing of Apollo 11, making the U.S. the first and only nation to ever land humans on the Moon. Simultaneously, the civil rights movement (1954–1968) led to court rulings and legislation abolishing codified racial discrimination against African-Americans. With the Soviet Union's collapse and the subsequent end of the Cold War in 1991, the United States emerged as the world's sole superpower. Following the September 11 attacks in 2001, the United States became a lead member of the Global War on Terrorism.

The United States government is a federal republic with three separate branches of government. It has a bicameral legislature composed of the House of Representatives, a lower house determined by state population; and the Senate, an upper house based on equal representation for each state. Many policy issues are decentralized, with widely differing laws by state. It is a representative democracy and has a market economy. The U.S. ranks very highly in international measures of quality of life, income and wealth, economic competitiveness, human rights, innovation, and education; it has low levels of perceived corruption and the highest median income per person of any polity in the world. It has high levels of incarceration and inequality and lacks universal health care. As a melting pot of cultures and ethnicities, the U.S. has been shaped by the world's largest immigrant population.

A highly developed country, the American economy accounts for approximately a quarter of global GDP and is the world's largest by GDP at market exchange rates. By value, the United States is the world's largest importer and second-largest exporter. Although it accounts for just over 4.2% of the world's total population, the U.S. holds over 30% of the total wealth in the world, the largest share held by any country. The United States is a founding member of the United Nations, World Bank, International Monetary Fund, Organization of American States, NATO, and is a permanent member of the United Nations Security Council. The U.S. is the foremost military power in the world and a dominant political, cultural, and scientific force internationally.