The United States of America (USA or U.S.A.), commonly referred to as the United States (US or U.S.), America, and sometimes the States, is a federal republic consisting of 50 states and a federal district. The 48 contiguous states and Washington, D.C., are in central North Americabetween Canada and Mexico. The state of Alaska is the northwestern part of North America and the state of Hawaii is an archipelago in the mid-Pacific. The country also has five populated and nine unpopulated territories in the Pacific and the Caribbean. At 3.71 million square miles (9.62 million km2) and with around 318 million people, the United States is the world's 3rd or 4th-largest country by total area and third-largest by 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 United States is also extremely diverse, and it is home to a wide variety of wildlife.
Paleo-Indians migrated from Eurasia to what is now the U.S. mainland around 15,000 years ago, with European colonization beginning in the 16th century.