Government Federal presidential constitutional republic
- President Barack Obama (D)
- Vice President Joe Biden (D)
- Speaker of the House John Boehner (R)
- Chief Justice John Roberts
- Upper house Senate
- Lower house House of Representatives
Independence from the Kingdom of Great Britain
- Declared July 4, 1776
- Recognized September 3, 1783
- Current constitution June 21, 1788
- Total 9,826,675 km2 (3rd/4th)
3,794,101 sq mi
- Water (%) 6.76
- 2012 estimate 313,201,000 (3rd)
- Density 33.7/km2
Currency United States dollar ($) (USD)
Drives on the right
a. English is the official language of at least 28 states—some sources give a higher figure, based on differing definitions of "official". English and Hawaiian are both official languages in the state of Hawaii.
b. English is the de facto language of American government and the sole language spoken at home by 80% of Americans age five and older. Spanish is the second most commonly spoken language.
c. Whether the United States or the People's Republic of China is larger is disputed. The figure given is from the U.S. Central Intelligence Agency's World Fact book. Other sources give smaller figures. All authoritative calculations of the country's size include only the 50 states and the District of Columbia, not the territories.
d. The population estimate includes people whose usual residence is in the fifty states and the District of Columbia, including noncitizens. It does not include either those living in the territories, amounting to more than 4 million U.S. citizens (most in Puerto Rico), or U.S. citizens living outside the United States.
The United States of America (also called 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 middle 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 seventh-highest.
Indigenous peoples descended from forebears who migrated from Asia have inhabited what is now the mainland United States for many thousands of years. This Native American population was greatly reduced by disease and warfare after European contact. The United States was founded by thirteen British colonies located along the Atlantic seaboard. On July 4, 1776, they issued the Declaration of Independence, which proclaimed their right to self-determination and their establishment of a cooperative union. The rebellious states defeated the British Empire in the American Revolution, the first successful colonial war of independence.  The current United States Constitution was adopted on September 17, 1787; its ratification the following year made the states part of a single republic with a strong central government. The Bill of Rights, comprising ten constitutional amendments guaranteeing many fundamental civil rights and freedoms, was ratified in 1791.
Through the 19th century, the United States displaced native tribes, acquired the Louisiana territory from France, Florida from Spain, part of the Oregon Country from the United Kingdom, Alta California and New Mexico from Mexico, and Alaska from Russia, and annexed the Republic of Texas and the Republic of Hawaii. Disputes between the agrarian South and industrial North over the expansion of the institution of slavery and states' rights provoked the Civil War of the 1860s. The North's victory prevented a permanent split of the country and led to the end of legal slavery in the United States. By the 1870s, its national economy was the world's largest. The Spanish–American War and World War I confirmed the country's status as a military power. It emerged from World War II as the first country with nuclear weapons and a permanent member of the United Nations Security Council. The end of the Cold War and the dissolution of the Soviet Union left the United States as the sole superpower. The country accounts for 41% of global military spending, and is a leading economic, political, and cultural force in the world.
In 1507, German cartographer Martin Waldseemüller produced a world map on which he named the lands of the Western Hemisphere "America" after Italian explorer and cartographer Amerigo Vespucci. The former British colonies first used the country's modern name in the 1776 Declaration of Independence, the "unanimous Declaration of the thirteen united States of America". On November 15, 1777, the Second Continental Congress adopted the Articles of Confederation, which states, "The Stile of this Confederacy shall be 'The United States of America'." The Franco-American treaties of 1778 used "United States of North America", but from July 11, 1778, "United States of America" was used on the country's bills of exchange, and it has been the official name ever since.
The short form "United States" is also standard. Other common forms include the "U.S.", the "USA", and "America". Colloquial names include the "U.S. of A." and, internationally, the "States". "Columbia", a once popular name for the United States, derives from Christopher Columbus; it appears in the name "District of Columbia".
The standard way to refer to a citizen of the United States is as an "American". Although "United States" is the official appositional term, "American" and "U.S." are more commonly used to refer to the country adjectivally ("American values", "U.S. forces"). "American" is rarely used in English to refer to people not connected to the United States.
The phrase "United States" was originally treated as plural—e.g., "the United States are"—including in the Thirteenth Amendment to the United States Constitution, ratified in 1865. It became common to treat it as singular—e.g., "the United States is"—after the end of the Civil War. The singular form is now standard; the plural form is retained in the idiom "these United States".
Geography and environment
The land area of the contiguous United States is approximately 1,900 million acres (7,700,000 km2). Alaska, separated from the contiguous United States by Canada, is the largest state at 365 million acres (1,480,000 km2). Hawaii, occupying an archipelago in the central Pacific, southwest of North America, has just over 4 million acres (16,000 km2). The United States is the world's third or fourth largest nation by total area (land and water), ranking behind Russia and Canada and just above or below China. The ranking varies depending on how two territories disputed by China and India are counted and how the total size of the United States is measured: calculations range from 3,676,486 square miles (9,522,055 km2) to 3,717,813 square miles (9,629,091 km2) to 3,794,101 square miles (9,826,676 km2). Including only land area, the United States is third in size behind Russia and China, just ahead of Canada.
The coastal plain of the Atlantic seaboard gives way further inland to deciduous forests and the rolling hills of the Piedmont. The Appalachian Mountains divide the eastern seaboard from the Great Lakes and the grasslands of the Midwest. The Mississippi–Missouri River, the world's fourth longest river system, runs mainly north–south through the heart of the country. The flat, fertile prairie of the Great Plains stretches to the west, interrupted by a highland region in the southeast. The Rocky Mountains, at the western edge of the Great Plains, extend north to south across the country, reaching altitudes higher than 14,000 feet (4,300 m) in Colorado. Farther west are the rocky Great Basin and deserts such as the Mojave. The Sierra Nevada and Cascade mountain ranges run close to the Pacific coast. At 20,320 feet (6,194 m), Alaska's Mount McKinley is the tallest peak in the country and in North America. Active volcanoes are common throughout Alaska's Alexander and Aleutian Islands, and Hawaii consists of volcanic islands. The supervolcano underlying Yellowstone National Park in the Rockies is the continent's largest volcanic feature.