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 self-governing territories, and various possessions. The 48 contiguous states and federal district are in central North America between Canada and Mexico, with the state of Alaska in the northwestern part of North America and the state of Hawaii comprising an archipelago in the mid-Pacific. The territories are scattered about the Pacific Ocean and the Caribbean Sea. At 3.8 million square miles (9.8 million km2) and with over 324 million people, the United States is the world's third largest country by total area (and 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 other countries. The geography and climate are also extremely diverse, and the country is home to a wide variety of wildlife.