The United States of America (commonly called the United States, the U.S., the USA, America, and the States) is a federal constitutional republic consisting of 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 mid-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 314 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.

Paleoindians migrated from Asia to what is now the United States mainland around 15,000 years ago. The descendent and isolated Native American population was greatly reduced by European contact, primarily by disease brought by explorers and traders. European colonization occurred, beginning about 1600, chiefly from England. The United States emerged from thirteen British colonies located along the Atlantic seaboard, which developed their own economies and democratic political systems. Patriots protested at British intrusions into taxation policies that Americans considered their rights, leading to war in 1775. On July 4, 1776, delegates from the 13 states to the Continental Congress unanimously issued the Declaration of Independence, which established the United States of America. The new nation, in alliance with France, defeated