The United States of America (USA), commonly referred to as the United 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 America between Canada and Mexico. The country also has five populated and numerous unpopulated territories in the Pacific and the Caribbean. The state of Alaska is located in the northwestern part of North America and the state of Hawaii is an archipelago in the mid-Pacific. At 3.8 million square miles and with over 320 million people, the United States is the world's fourth-largest country by total area and 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 of the United States are also extremely diverse, and the country is home to a wide variety of wildlife.