Officially UNITED STATES OF AMERICA, abbreviations U.S. or U.S.A., byname AMERICA country of North America, a federal republic of 50 states. Besides the 48 contiguous states that occupy the middle latitudes of the continent, the United States includes the state of Alaska, at the northwestern extreme of North America, and the island state of Hawaii, in the mid-Pacific Ocean. The coterminous states are bounded on the north by Canada, on the east by the Atlantic Ocean, on the south by the Gulf of Mexico and Mexico, and on the west by the Pacific Ocean. The national capital is Washington, which is coextensive with the District of Columbia, the federal capital region created in 1790.

The total area of the United States is 3,679,192 square miles (9,529,063 square kilometres), making it the fourth largest country in the world in area (after Russia, Canada, and China). Outlying territories and other politically associated areas in the Pacific Ocean and the Caribbean Sea add approximately 4,000 square miles to this figure.

The major characteristic of the United States is probably its great variety. Its physical environment ranges from the Arctic to the subtropical, from the moist rain forest to the arid desert, from the rugged mountain peak to the flat prairie. Although the total population of the United States is large by world standards, its overall population density is relatively low; the country embraces some of the world's largest urban concentrations as well as some of the most extensive areas that are almost devoid of habitation.

The United States contains a highly diverse population; but, unlike a country such as China that largely incorporated indigenous peoples, its diversity has to a great degree come from an immense and sustained global immigration. Probably no other country has a wider range of racial, ethnic, and cultural types than does the United States. In addition to the presence of surviving native Americans (including American Indians, Aleuts, and Eskimo) and the descendants of Africans taken as slaves to America, the national character has been enriched, tested, and constantly redefined by the tens of millions of immigrants who by and large have gone to America hoping for greater social, political, and economic opportunities than they had in the places they left.

The United States is the world's greatest economic power, measured in terms of gross national product (GNP). The nation's wealth is partly a reflection of its rich natural resources and its enormous agricultural output, but it owes more to the country's highly developed industry. Despite its relative economic self-sufficiency in many areas, the United States is the most important single factor in world trade by virtue of the sheer size of its economy. Its exports and imports represent major proportions of the world total. The United States also impinges on the global economy as a source of and as a destination for investment capital. The country continues to sustain an economic life that is more diversified than any other on Earth, providing the majority of its people with one of the world's highest standards of living.

The United States is relatively young by world standards, being barely more than 200 years old; it achieved its current size only in the mid-20th century. America was the first of the European colonies to separate successfully from its motherland, and it was the first nation to be established on the premise that sovereignty rests with its citizens and not with the government. In its first century and a half, the country was mainly preoccupied with its own territorial expansion and economic growth and with social debates that ultimately led to civil war and a healing period that is still not complete. In the 20th century the United States emerged as a world power, and since World War II it has been one of the preeminent powers. It has not accepted this mantle easily nor always carried it willingly; the principles and ideals of its founders have been tested by the pressures and exigencies of its dominant status. Although the United States still offers its residents opportunities for unparalleled personal advancement and wealth, the depletion of its resources, contamination of its environment, and continuing social and economic inequality that perpetuates areas of poverty and blight all threaten the fabric of the country.