The United States of America (USA), commonly referred to as the United States (U.S.) or America, is a federal republic composed of 50 states, the federal district of Washington, D.C., five major territories, and various possessions. The 48 contiguous states and Washington, D.C. are in central North America between Canada and Mexico. The state of Alaska is in the northwestern part of North America and the state of Hawaii is an archipelago in the mid-Pacific. The territories are scattered about the Pacific Ocean and the Caribbean Sea. At 3.8 million square miles (9.9 million km2) and with over 320 million people, the country is the world's third largest by total area (fourth largest by land area) and the third most populous. 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 are also extremely diverse, and the country is home to a wide variety of wildlife.