Main articles: Geography of the United States and Territorial evolution of the United States

Topographic map of the contiguous United States

Climate zones of the contiguous United States

The United States is situated almost entirely in the western hemisphere: the contiguous United States stretches from the Pacific on the west to the Atlantic on the east, with the Gulf of Mexico to the southeast, and bordered by Canada on the north and Mexico on the south. Alaska is the largest state in area; separated from the contiguous U.S. by Canada, it touches the Pacific on the south and Arctic Ocean on the north. Hawaii occupies an archipelago in the central Pacific, southwest of North America. The United States is the world’s third or fourth largest nation by total area, before or after China. The ranking varies depending on (a) how two territories disputed by China and India are counted and (b) how the total size of the United States is calculated: the CIA World Factbook gives 9,826,630 km² (3,794,083 sq mi),^[1] the United Nations Statistics Division gives 9,629,091 km² (3,717,813 sq mi),^[14] and the Encyclopedia Britannica gives 9,522,055 km² (3,676,486 sq mi).^[15] Including only land area, the United States is third in size behind Russia and China, just ahead of Canada.^[16] The United States also possesses several insular territories scattered around the West Indies (e.g., the commonwealth of Puerto Rico) and the Pacific (e.g., Guam).

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 land of the Great Plains stretches to the west. The Rocky Mountains, at the western edge of the Great Plains, extend north to south across the continental United States, reaching altitudes higher than 14,000 feet (4,300 m) in Colorado.^[17] The area to the west of the Rocky Mountains is dominated by the rocky Great Basin and deserts such as the Mojave. The Sierra Nevada range runs parallel to the Rockies, relatively close to the Pacific coast. At 20,320 feet (6,194 m), Alaska’s Mount McKinley is the country’s tallest peak. Active volcanoes are common throughout the Alexander and Aleutian Islands, and the entire state of Hawaii is built upon tropical volcanic islands. The supervolcano underlying Yellowstone National Park in the Rockies is the continent’s largest volcanic feature.^[18]

Because of the United States’ large size and wide range of geographic features, nearly every type of climate is represented. The climate is temperate in most areas, tropical in Hawaii and southern Florida, polar in Alaska, semi-arid in the Great Plains west of the 100th meridian, desert in the Southwest, Mediterranean in Coastal California, and arid in the Great Basin. Extreme weather is not uncommonthe states bordering the Gulf of Mexico are prone to hurricanes, and most of the world’s tornadoes occur within the continental United States, primarily in the Midwest.^[19]

The term America, for the lands of the western hemisphere, was coined in the early sixteenth century after Amerigo Vespucci, an Italian explorer and cartographer. The full name of the country was first used officially in the Declaration of Independence, which was the “unanimous Declaration of the thirteen united States of America” adopted by the “Representatives of the united States of America” on July 4, 1776.^[11] The current name was finalized on November 15, 1777, when the Second Continental Congress adopted the Articles of Confederation, the first of which states, “The Stile of this Confederacy shall be ‘The United States of America.’” Common short forms and abbreviations of the United States of America include the United States, the U.S., the U.S.A., and America. Colloquial names for the country include the U.S. of A. and the States. Columbia, a once popular name for the Americas and the United States, was derived from Christopher Columbus. It appears in the name “District of Columbia”. A female personification of Columbia appears on some official documents, including certain prints of U.S. currency.

The standard way to refer to a citizen of the United States is as an American. Though United States is the formal adjective, American and U.S. are the most common adjectives used to refer to the country (”American values,” “U.S. forces”). American is rarely used in English to refer to people not connected to the United States.^[12]

The phrase “the United States” was originally treated as plurale.g, “the United States are”including in the Thirteenth Amendment to the Constitution, ratified in 1865. However, it became increasingly common to treat the name as singulare.g., “the United States is”after the end of the Civil War. The singular form is now standard, while the plural form is retained in the set idiom “these United States.”^[13]

The United States of America is a constitutional federal 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 its east and Russia to the west across the Bering Strait, and the state of Hawaii is an archipelago in the mid-Pacific. The United States also possesses several territories, or insular areas, scattered around the Caribbean and Pacific.

At 3.79 million square miles (9.83 million km²) and with more than 300 million people, the United States is the third or fourth largest country by total area, and third largest by land area and by population. The United States is one of the world’s most ethnically diverse nations, the product of large-scale immigration from many countries.^[7] The U.S. economy is the largest national economy in the world, with a nominal 2006 gross domestic product (GDP) of more than US$13 trillion (over 19% of the world total based on purchasing power parity).^[4][8]

The nation was founded by thirteen colonies of Great Britain located along the Atlantic seaboard. Proclaiming themselves “states,” they issued the Declaration of Independence on July 4, 1776. The rebellious states defeated Great Britain in the American Revolutionary War, the first successful colonial war of independence.^[9] A federal convention adopted the current United States Constitution on September 17, 1787; its ratification the following year made the states part of a single republic. The Bill of Rights, comprising ten constitutional amendments, was ratified in 1791.

In the nineteenth century, the United States acquired land from France, Spain, the United Kingdom, Mexico, and Russia, and annexed the Republic of Texas and the Republic of Hawaii. Disputes between the agrarian South and industrial North over states’ rights and the expansion of the institution of slavery provoked the American Civil War of the 1860s. The North’s victory prevented a permanent split of the country and led to the end of slavery in the United States. The Spanish-American War and World War I confirmed the nation’s status as a military power. In 1945, the United States emerged from World War II as the first country with nuclear weapons, a permanent member of the United Nations Security Council, and a founding member of NATO. In the post–Cold War era, the United States is the only remaining superpoweraccounting for approximately 50% of global military spendingand a dominant economic, political, and cultural force in the world.^[10]