The United States of America, also known as the United States, the U.S., the U.S.A., the U.S. of A., the States, and America is a country in North America. A federal republic, the United States shares land borders with Canada and Mexico, and extends from the Atlantic Ocean to the Pacific Ocean. Its capital is Washington, D.C.

The present-day continental United States has been inhabited for at least 15,000 years by indigenous tribes. After 16th-century European exploration and settlement, the English established new colonies, and gained control of others, in the eastern portion of the continent in the 17th and early 18th centuries. On 4 July 1776, at war with Britain over fair governance, thirteen of these colonies declared their independence; in 1783, the war ended in British acceptance of the new nation. Since then, the country has more than quadrupled in size: it now consists of 50 states, one federal district, and a number of overseas territories.

At over 3.7 million square miles (over 9.5 million km²), the U.S. is the third or fourth largest country by area, depending on the reckoning of the disputed areas of China. It is also the world's third most populous nation, with nearly 300 million people.

The United States has maintained a liberal democratic political system since it adopted its Articles of Confederation on 1 March 1781 and the Constitution, the Articles' replacement, on 17 September 1787. American military and economic influence increased throughout the 20th century; with the collapse of the Soviet Union at the end of the Cold War, the nation emerged as the world's sole remaining superpower.