The United States of America (commonly referred to as the United States, the U.S., the USA, or America) is a federal constitutional republic comprising fifty states and a federal district. The country is situated mostly in central North America, where its 48 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. The state of Hawaii is an archipelago in the mid-Pacific. The country also possesses several territories, or insular areas, in the Caribbean and Pacific.At 3.79 million square miles (9.83 million km2) and with about 307 million people, the United States is the third or fourth largest country by total area, and the third largest by land area and population. The United States 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 largest national economy in the world, with an estimated 2008 gross domestic product (GDP) of US $14.3 trillion (23% of the world total based on nominal GDP and almost 21% at purchasing power parity).The nation was founded by thirteen colonies of Great Britain located along the Atlantic seaboard. On July 4, 1776, they issued the Declaration of Independence, which proclaimed their independence from Great Britain and their formation of a cooperative union. The rebellious states defeated Great Britain in the American Revolutionary War, the first successful colonial war of independence The Philadelphia Convention adopted the current United States Constitution 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.

English is the de facto national language. Although there is no official language at the federal level, some laws—such as U.S. naturalization requirements—standardize English. In 2005, about 216 million, or 81% of the population aged five years and older, spoke only English at home. Spanish, spoken by 12% of the population at home, is the second most common language and the most widely taught second language. Some Americans advocate making English the country's official language, as it is in at least twenty-eight states. Both Hawaiian and English are official languages in Hawaii by state law. While neither has an official language, New Mexico has laws providing for the use of both English and Spanish, as Louisiana does for English and French. Other states, such as California, mandate the publication of Spanish versions of certain government documents including court forms. Several insular territories grant official recognition to their native languages, along with English: Samoan and Chamorro are recognized by American Samoa and Guam, respectively; Carolinian and Chamorro are recognized by the Northern Mariana Islands; Spanish is an official language of Puerto Rico.

The United States is a multicultural nation, home to a wide variety of ethnic groups, traditions, and values. Aside from the now small Native American and Native Hawaiian populations, nearly all Americans or their ancestors immigrated within the past five centuries. The culture held in common by most Americans—mainstream American culture—is a Western culture largely derived from the traditions of European immigrants with influences from many other sources, such as traditions brought by slaves from Africa. More recent immigration from Asia and especially Latin America has added to a cultural mix that has been described as both a homogenizing melting pot and a heterogeneous salad bowl in which immigrants and their descendants retain distinctive cultural characteristics. According to Geert Hofstede's cultural dimensions analysis, the United States has the highest individualism score of any country studied. While the mainstream culture holds that the United States is a classless society, scholars identify significant differences between the country's social classes, affecting socialization, language, and values.[ The American middle and professional class has initiated many contemporary social trends such as modern 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, plays a key role in attracting immigrants, some analysts find that the United States has less social mobility than Western Europe and Canada. Women now mostly work outside the home and receive a majority of bachelor's degrees. In 2005, 28% of households were married childless couples, the most common arrangement. Same-sex marriage is contentious. Several states permit civil unions in lieu of marriage. Since 2003, four state supreme courts have ruled bans on same-sex marriage unconstitutional, while voters in more than a dozen states approved constitutional bans on the practice. In 2009, Vermont, Maine, and New Hampshire became the first states to permit same-sex marriage through legislative action.

Mainstream American culinary arts are similar to those in other Western countries. Wheat is the primary cereal grain. Traditional American cuisine uses ingredients such as turkey, white-tailed deer venison, potatoes, sweet potatoes, corn, squash, and maple syrup, indigenous foods employed by Native Americans and early European settlers. Slow-cooked pork and beef barbecue, crab cakes, potato chips, and chocolate chip cookies are distinctively American styles. Soul food, developed by African slaves, is popular around the South and among many African Americans elsewhere. Syncretic cuisines such as Louisiana creole, Cajun, and Tex-Mex are regionally important. Characteristic dishes such as apple pie, fried chicken, pizza, hamburgers, and hot dogs derive from the recipes of various immigrants. French fries, Mexican dishes such as burritos and tacos, and pasta dishes freely adapted from Italian sources are widely consumed. Americans generally prefer coffee to tea. Marketing by U.S. industries is largely responsible for making orange juice and milk ubiquitous breakfast beverages. During the 1980s and 1990s, Americans' caloric intake rose 24%;frequent dining at fast food outlets is associated with what health officials call the American "obesity epidemic." Highly sweetened soft drinks are widely popular; sugared beverages account for 9% of the average American's caloric intake.

Since the late 19th century, baseball has been regarded as the national sport; American football, basketball, and ice hockey are the country's three other leading professional team sports. College football and basketball attract large audiences. Football is now by several measures the most popular spectator sport. Boxing and horse racing were once the most watched individual sports, but they have been eclipsed by golf and auto racing, particularly NASCAR. Soccer is played widely at the youth and amateur levels. Tennis and many outdoor sports are popular as well.While most major U.S. sports have evolved out of European practices, basketball, volleyball, skateboarding, snowboarding, and cheerleading are American inventions. Lacrosse and surfing arose from Native American and Native Hawaiian activities that predate Western contact. Eight Olympic Games have taken place in the United States. The United States has won 2,301 medals at the Summer Olympic Games, more than any other country, and 216 in the Winter Olympic Games, the second most.