The United States of America (commonly referred as the United States, the U.S., the USA, or America) 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.

Geography
The eastern United States has a varied topography. A broad, flat coastal plain lines the Atlantic and Gulf shores from the Texas-Mexico border to New York City, and includes the Florida peninsula. Areas further inland feature rolling hills and temperate forests. The Appalachian Mountains form a line of low mountains separating the eastern seaboard from the Great Lakes and the Mississippi Basin. The five Great Lakes are located in the north-central portion of the country, four of them forming part of the border with Canada. The southeast of the United States contain subtropical forests and, near the gulf coast, mangrove wetlands, especially in Florida. West of the Appalachians lies the Mississippi River basin and two large eastern tributaries, the Ohio River and the Tennessee River. The Ohio and Tennessee Valleys and the Midwest consist largely of rolling hills and productive farmland, stretching south to the Gulf Coast.

Climate
Due to its large size and wide range of geographic features, the United States contains examples of nearly every global climate. The climate is temperate in most areas, subtropical in the Deep South, tropical in Hawaii and southern Florida, polar in Alaska, semiarid in the Great Plains west of the 100th meridian, Mediterranean in coastal California and arid in the Great Basin. It’s comparatively favorable agricultural climate contributed (in part) to the country's rise as a world power, with infrequent severe drought in the major agricultural regions, a general lack of widespread flooding, and a mainly temperate climate that receives adequate precipitation.

Population
The current U.S.A. population is over 300 million people (303,500,000 in early 2008) so the United States has the world's third largest population (following China and India).As the world's population is approximately 6.6 billion, the current United States population represents a mere 4.5% of the world's population so about one in every twenty people on the planet is a resident of the United States of America.
The American population more than tripled during the 20th century—a growth rate of about 1.3% a year—from about 76 million in 1900 to 281 million in 2000. It reached the 200 million mark in 1967 and the 300 million mark on October 17, 2006. Currently, population growth is fastest among minorities as a whole, and according to the Census Bureau's estimation for 2005, 45% of American children under the age of 5 belonged to minority groups.
Hispanic and Latino Americans accounted for almost half (1.4 million) of the national population growth of 2.9 million between July 1, 2005, and July 1, 2006. Immigrants and their U.S.-born descendants are expected to provide most of the U.S. population gains in the decades ahead. Since the liberalization of immigration policy in 1965, the number of first-generation immigrants living in the United States has quadrupled, from 9.6 million in 1970 to about 38 million in 2007. Almost 97% of residents of the 10 largest American cities in 1900 were non-Hispanic whites. In 2006, non-Hispanic whites were the minority in thirty-five of the fifty largest cities.

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