The United States of America, usually referred to as the United States, USA, the U.S. or America, is a country in North America and other insular islands in the Pacific Ocean with its capital Washington, D.C. in the District of Columbia and a constitutional federal republic comprising fifty states and a federal district, as well as several territories, or insular areas, scattered around the Caribbean and Pacific. 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.

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.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 25% of the world total based on nominal GDP and almost 20% by purchasing power parity).

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.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 legal 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 superpower—accounting for approximately 50% of global military spending—and a dominant economic, political, and cultural force in the world.

Culture
The United States is a multicultural nation, home to a wide variety of ethnic groups, traditions, and values.There is no "American" ethnicity; aside from the now relatively small Native American population, nearly all Americans or their ancestors immigrated within the past five centuries.[196] The culture held in common by the majority of Americans is referred to as mainstream American culture, a Western culture largely derived from the traditions of Western European migrants, beginning with the early English and Dutch settlers. German, Irish, and Scottish cultures have also been very influential.Certain cultural attributes of Mandé and Wolof slaves from West Africa were adopted by the American mainstream; based more on the traditions of Central African Bantu slaves, a distinct African American culture developed that would eventually have a major effect on the mainstream as well.[197] Westward expansion integrated the Creoles and Cajuns of Louisiana and the Hispanos of the Southwest and brought close contact with the culture of Mexico. Large-scale immigration in the late nineteenth and early twentieth centuries from Southern and Eastern Europe introduced many new cultural elements. More recent immigration from Asia and especially Latin America has had broad impact. The resulting mix of cultures may be characterized as a homogeneous melting pot or as a pluralistic salad bowl in which immigrants and their descendants retain distinctive cultural characteristics.

While American culture maintains that the United States is a classless society,economists and sociologists have identified cultural differences between the country's social classes, affecting socialization, language, and values.The American middle and professional class has been the source of many contemporary social trends such as feminism, environmentalism, and multiculturalism.Americans' self-images, social viewpoints, and cultural expectations are associated with their occupations to an unusually close degree.While Americans tend greatly to value socioeconomic achievement, being ordinary or average is generally seen as a positive attribute.Though the American Dream, or the perception that Americans enjoy high social mobility, played a key role in attracting immigrants, particularly in the late 1800s,some analysts find that the United States has less social mobility than Western Europe and Canada.

Women, many of whom were formerly more limited to domestic roles, now mostly work outside the home and receive a majority of bachelor's degrees.The changing role of women has also changed the American family. In 2005, no household arrangement defined more than 30% of households; married childless couples were most common, at 28%.The extension of marital rights to homosexual persons is an issue of debate; several more liberal states permit civil unions in lieu of marriage. In 2003, the Massachusetts Supreme Judicial Court ruled that state's ban on same-sex marriage unconstitutional,the Supreme Court of California ruled similarly in 2008.Forty-three states still legally restrict marriage to the traditional man-and-woman model.

Education
American public education is operated by state and local governments, regulated by the United States Department of Education through restrictions on federal grants. Children are required in most states to attend school from the age of six or seven (generally, kindergarten or first grade) until they turn eighteen (generally bringing them through 12th grade, the end of high school); some states allow students to leave school at sixteen or seventeen.About 12% of children are enrolled in parochial or nonsectarian private schools. Just over 2% of children are homeschooled. The United States has many competitive private and public institutions of higher education, as well as local community colleges of varying quality with open admission policies. Of Americans twenty-five and older, 84.6% graduated from high school, 52.6% attended some college, 27.2% earned a bachelor's degree, and 9.6% earned graduate degrees.The basic literacy rate is approximately 99%. The United Nations assigns the United States an Education Index of 0.97, tying it for twelfth-best in the world.