United States, officially United States of America, abbreviated U.S. or U.S.A., is a country in North America; a federal republic of 50 states. It is the fourth largest country in the world in area (after Russia, Canada, and China). 

The major characteristic of the United States is probably its great variety. Its physical environment ranges from the Arctic to the subtropical, from the moist rain forest to the arid desert, from the rugged mountain peak to the flat prairie. Although the total population of the United States is large by world standards, its overall population density is relatively low.

The United States contains a highly diverse population. Its diversity, to a great degree, has come from an immense and sustained global immigration.  Probably no other country has a wider range of racial, ethnic and cultural types than does the United States.