The United States of America (USA or U.S.A.), commonly called the United States (US or U.S.) and America, is a federal republic consisting of fifty states and a federal district. The 48 contiguous states and the federal district of Washington, D.C. are in central North America between Canada and Mexico. The state of Alaska is west of Canada and east of Russia across the Bering Strait, and the state of Hawaii is in the mid-North Pacific. The country also has five populated and nine unpopulated territories in the Pacific and the Caribbean.At 3.79 million square miles (9.83 million km2) in total and with around 315 million people, the United States is the third largest country 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 geography and climate of the U.S. is also extremely diverse, with deserts, plains, forests, and mountains that are also home to a wide variety of wildlife.