The United States of America is a federal constitutional republic comprising fifty states, one federal district, and fourteen territories. The country is situated almost entirely in the western hemisphere: its forty-eight contiguous states and Washington, D.C., the capital district, lie in central North America 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 the state of Hawaii is in the mid-Pacific. U.S. territories, or insular areas, are scattered around the Caribbean and Pacific.

At 3.7 million square miles (9.6 million km²) and with 300 million people, the United States is the third or fourth largest country by total area, and third largest by land area and population. The United States is one of the world's most ethnically diverse nations, the product of large-scale immigration from many countries. Its national economy is the world's largest, with a nominal 2006 gross domestic product (GDP) of more than $13 trillion.

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 seceding states defeated Britain in the American Revolutionary War. 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, Mexico, and Russia, and annexed the Republic of Texas and the Republic of Hawaii. The American Civil War ended slavery in the United States and prevented a permanent split of the country. The Spanish-American War and World War I confirmed its status as a military power. In 1945, the United States emerged from World War II as the first country with nuclear weapons and a permanent member of the United Nations Security Council. The sole remaining superpower in the post–Cold War era, it is the dominant economic, political, cultural, and military force in the world.

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 obliged 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 percent of children are enrolled in parochial or nonsectarian private schools. Just over 2 percent of children are home schooled. The United States has many competitive private and public institutions of higher education; 168 U.S. universities are in the world's top 500, 17 in the top 20. There are also many smaller universities and liberal arts colleges, and local community colleges of varying quality with open admission policies. The United States has a basic literacy rate of approximately 99 percent. Of Americans twenty-five and older, 84.6 percent graduated from high school, 52.6 percent attended some college, 27.2 percent earned a bachelor's degree, and 9.6 percent earned graduate degrees. The United Nations assigns the United States an Education Index of 99.9, tieing it with twenty other nations for the top score.

The American life expectancy of seventy-eight years at birth is a year shorter than the overall figure in Western Europe, and three to four years lower than that of Norway and Switzerland. The infant mortality rate of 6.37 per thousand places the United States 41st out of 221 countries, likewise behind most of Western Europe. Approximately one-third of the adult population is obese and an additional third is overweight; the obesity rate, the highest in the industrialized world, has more than doubled in the last quarter-century. Obesity-related type 2 diabetes is considered epidemic by healthcare professionals. The U.S. adolescent pregnancy rate, 79.8 per 1,000 women, is nearly four times that of France and five times that of Germany. Abortion, legal on demand, is a source of great political controversy. Many states ban public funding of the procedure and have laws to restrict late-term abortions, require parental notification for minors, and mandate a waiting period prior to treatment. Geographical access to abortion is limited: 87 percent of U.S. counties have no abortion provider. Nonetheless, while the incidence of abortion is in decline, the U.S. abortion ratio of 241 per 1,000 live births and abortion rate of 15 per 1,000 women aged 15–44 remain higher than those of most Western nations.