The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a transcontinental 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 with limited sovereignty. It is the third-largest country by both land and total area. The United States shares land borders with Canada to the north and with Mexico to the south as well as maritime borders with the Bahamas, Cuba, and Russia, among others. With more than 331 million people, it is the third most populous country in the world. The national capital is Washington, D.C., and the most populous city and financial center is New York City. It is known for its highly diverse climate and geography, encompassing landscapes that range from polar to tropical, arid to humid, and is officially recognized as one of the 17 megadiverse countries.

Paleo-Indians migrated from Siberia to the North American mainland at least 12,000 years ago, and European colonization began in the 16th century. The United States emerged from the Thirteen British Colonies established along the East Coast. Disputes with Great Britain over taxation and political representation led to the American Revolutionary War (1775–1783), which established the nation's independence. In the late 18th century, the U.S. began expanding across North America, gradually obtaining new territories, sometimes through war, frequently displacing Native Americans, and admitting new states. This was strongly related to belief in manifest destiny, and by 1848, the United States spanned the continent from east to west. Slavery was legal in the southern United States until 1865, when the American Civil War led to its abolition. A century later, the civil rights movement led to legislation outlawing racial discrimination against African Americans. The Spanish–American War and World War I established the U.S. as a world power, and the aftermath of World War II left the United States and the Soviet Union as the world's two superpowers. During the Cold War, both countries opposed each other in the Korean and Vietnam Wars but avoided direct military conflict. They also competed in the Space Race, which culminated in the 1969 American spaceflight that first landed humans on the Moon. The Soviet Union's dissolution in 1991 ended the Cold War, leaving the United States as the world's sole superpower. The September 11 attacks in 2001 resulted in the United States launching the war on terror, which included the War in Afghanistan (2001–2021) and the Iraq War (2003–2011).

The United States is a federal republic with three separate branches of government, including a bicameral legislature. It is a founding member of the United Nations, World Bank, International Monetary Fund, Organization of American States, NATO, and other international organizations. It is a permanent member of the United Nations Security Council. Considered a melting pot of cultures and ethnicities, its population has been profoundly shaped by centuries of immigration. The United States is a liberal democracy; it ranks high in international measures of human rights, quality of life, income and wealth, economic freedom, and education; and it has low levels of perceived corruption. It lacks universal health care, retains capital punishment, and has high levels of incarceration and inequality.

The United States is a highly developed country, and its 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. Making up more than a third of global military spending, it is the foremost military power in the world and a leading political, cultural, and scientific force.