The United States of America also called the United States, the US or America, is the third largest country. It is a federal constitutional republic comprising fifty states and a federal district. The country is situated mostly in central North America, where the forty-eight contiguous states and Washington ZIP, the capital region, located between the Pacific and Atlantic Ocean, bordering with Canada to the north and Mexico at 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 Pacific means. The country also possesses several territories in the Pacific and the Caribbean. In 1507, German cartographer Martin Waldseemüller created a world map on which he named the chunk of dry Western Hemisphere "America" by Italian explorer and cartographer Americo Vespucci. The former British colonies first used the modern name of the country in the Declaration of Independence of 1776. With an area of 9.83 million square kilometers and more than 320 million inhabitants, the United States is the third or fourth largest country in total area, and third largest in land area and population. It is one of the most multicultural nations of the world, large-scale migration product of many countries. The US economy is the largest national economy in the world, with an estimated GDP in 2011 $ 15.1 trillion. The official currency is the United States dollar ($,USD)