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 50 states and a federal district. Washington, D.C., has been the capital of the United States since 1800. Eight other cities have served as the meeting place for the U.S. Congress and are therefore considered to have once been the capital of the United States. In addition, each of the fifty U.S. states and the five principal territories of the United States maintains its own capital.
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”. In the American federalist system, citizens are usually subject to three levels of government: federal, state, and local.
The United States is a developed country and has the world’s largest national economy, with an estimated GDP in 2013 of $16.7 trillion – 23% of global nominal GDP and 19% at purchasing-power parity.