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The United States of America (USA), commonly referred to as the United States (U.S.) or America, is a federal republic composed of 50 states, the federal district of Washington, D.C., five major territories, and various possessions. Driven by the doctrine of Manifest Destiny, the United States embarked on a vigorous expansion across North America throughout the 19th century. The United States is the world's oldest surviving federation. It is a constitutional republic and representative democracy, "in which majority rule is tempered by minority rights protected by law". The federal government is composed of three branches: Legislative, Executive and the Judicial.
The land area of the contiguous United States is 2,959,064 square miles (7.7 Mm2). The United States is the world's third or fourth largest nation by total area. The country lies between 37.0902° N, 95.7129° W. The United States, with its large size and geographic variety, includes most climate types.
The U.S. Census Bureau estimated the country's population to be 323 425 550 as of April 25, 2016. The United States has a capitalist mixed economy. The U.S. GDP of $16.8 trillion constitutes 24% of the gross world product at market exchange rates and over 19% of the gross world product at purchasing power parity (PPP). The United States is the largest importer of goods and second largest exporter, though exports per capita are relatively low.
There are a total of 357 Nobel Laureates in USA. The United States has been a leader in scientific research and technological innovation since the late 19th century. The U.S. is a global leader in medical innovation. America solely developed or contributed significantly to 9 of the top 10 most important medical innovations. University of California San Francisco is the top most university with extensive research on Orthopedics.