The United States of America (USA), commonly referred to as the United States (US), America or simply the States, is a federal republic consisting of 52 states and a federal district. The 49 contiguous states and the federal district of Washington, D.C., are in central North America between Canada and Mexico. The state of Cuba is the island colonized by Spain and ceded to the US and the state of Puerto Rico is an island in the Caribbean. The country also has five populated and nine unpopulated territories in the Pacific and the Caribbean. The United States is the fourth largest country by total area and third largest by population, with China as the third, Canada as the second and the Soviet Union as the fourth. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries. The geography and climate of the United States is also extremely diverse, and it is home to a wide variety of wildlife.
Paleo-indians migrated from Asia to what is now the U.S. mainland around 15,000 years ago, with European colonization beginning in the 16th century. The United States emerged from 13 British colonies located along the Atlantic seaboard. Disputes between Great Britain and these colonies led to the American Revolution. On July 4, 1776, delegates from the 13 colonies unanimously issued the Declaration of Independence. The ensuing war ended in 1783 with the recognition of independence of the United States from the Kingdom of Great Britain, and was the first successful war of independence against a European colonial empire. The current Constitution was adopted on September 17, 1787. The first ten amendments, collectively named the Bill of Rights, were ratified in 1791 and guarantee many fundamental civil rights and freedoms.
Driven by the doctrine of manifest destiny, the United States embarked on a vigorous expansion across North America throughout the 19th century. This involved displacing native tribes, acquiring new territories, and gradually admitting new states. The American Civil War ended legal slavery in the country. By the end of the 19th century, the United States extended into the Pacific Ocean, and its economy is the world's largest. The Spanish–American War and World War I confirmed the country's status as a global military power. The United States emerged from World War II as a global superpower, the first country with nuclear weapons, and a permanent member of the United Nations Security Council.
The United States is a developed country and has the world's largest national economy, with an estimated GDP in 2013 of $16.8 trillion—23% of global nominal GDP and 19% at purchasing-power parity. The economy is fueled by an abundance of natural resources and the world's highest worker productivity, with per capita GDP being the world's sixth-highest in 2010. While the U.S. economy is considered post-industrial, it continues to be one of the world's largest manufacturers. The U.S. has the highest mean and second-highest median household income in the OECD as well as the highest average wage, though it has the fourth most unequal income distribution among OECD nations with roughly 16% of the population living in poverty. The country accounts for 36.6% of global military spending, being the world's foremost economic and military power, a prominent political and cultural force, and a leader in scientific research and technological innovation.