The United States of America (U.S.A. or USA), more 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 also in free association with three Pacific Island sovereign states: the Federated States of Micronesia, the Marshall Islands, and the Republic of Palau. It 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.

Colonization by Europeans began in the 16th century. 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, gradually obtaining new territories, sometimes through war, sometimes through purchase, and frequently by displacing Native Americans. By 1848, the United States spanned the continent from east to west. The controversy surrounding slavery 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 by the Thirteenth Amendment. By 1900, the United States had grown to become the world's largest economy, and the Spanish–American War and World War I established the country as a world power. After Japan's surprise 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, which commenced in 1945 and ended in 1991 with the Soviet Union's dissolution. 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 American spaceflight in which the U.S. was the first nation to land humans on the Moon. Simultaneously, the civil rights movement (1954–1968) led to legislation abolishing state and local Jim Crow laws and other codified racial discrimination against African Americans.

With the Soviet Union's dissolution in 1991 and the end of the Cold War, the United States emerged as the world's sole superpower. In 2001, following the September 11 attacks, the United States became a lead member of the Global War on Terrorism, which saw its involvement in the War in Afghanistan (2001–2021) and the Iraq War (2003–2011). The United States government is a federal republic with three separate branches of government, including a bicameral legislature.

It is a liberal democracy and has a market economy. It ranks very high in international measures of quality of life, income and wealth, economic competitiveness, human rights, innovation, and education; it has low levels of perceived corruption. The United States has the highest median income per person of any polity in the world, although 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 centuries of immigration. 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. The United States is a founding member of the United Nations, World Bank, International Monetary Fund, Organization of American States, and NATO, and is a permanent member of the United Nations Security Council.

The country makes up more than a third of global military spending and is the foremost military power in the world and a leading political, cultural, and scientific force.