The United States of America (commonly referred to as the United States, the U.S., the USA, or America əˈmɛərɪkə) is a federal constitutional republic comprising fifty states and a federal district. The country is situated mostly in central North America, where its forty-eight contiguous states and Washington, D.C., the capital district, lie between the Pacific and Atlantic Oceans, bordered by Canada to the north and Mexico to the south. The state of Alaska is in the northwest of the continent, with Canada to the east and Russia to the west across the Bering Strait. The state of Hawaii is an archipelago in the mid-Pacific. The country also possesses several territories in the Caribbean and Pacific.
At 3.79 million square miles (9.83 million km2) and with about 309 million people, the United States is the third or fourth largest country by total area, and the third largest both by land area and population. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries. The U.S. economy is the largest national economy in the world, with an estimated 2008 gross domestic product (GDP) of US $14.4 trillion (a quarter of nominal global GDP and a fifth of global GDP at purchasing power parity).