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, a federal district, 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 (9.842 million km2) and with over 320 million people, the country is the world’s third or fourth-largest by total 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 of the United States are also extremely diverse, and the country is home to a wide variety of wildlife.
The land area of the contiguous United States is 2,959,064 square miles (7.7 Mm2). Alaska, separated from the contiguous United States by Canada, is the largest state at 663,268 square miles (1.7 Mm2). Hawaii, occupying an archipelago in the central Pacific, southwest of North America, is 10,931 square miles (28,311 km2) in area. The populated territories of Puerto Rico, American Samoa, Guam, Northern Mariana Islands, and US Virgin Islands together cover 9,185 square miles (23,789 km2)
The coastal plain of the Atlantic seaboard gives way further inland to deciduous forests and the rolling hills of the Piedmont. The Appalachian Mountains divide the eastern seaboard from the Great Lakes and the grasslands of the Midwest. The Mississippi–Missouri River, the world’s fourth longest river system, runs mainly north–south through the heart of the country. The flat, fertile prairie of the Great Plains stretches to the west, interrupted by a highland region in the southeast.
The Rocky Mountains, at the western edge of the Great Plains, extend north to south across the country, reaching altitudes higher than 14,000 feet (4,300 m) in Colorado. Farther west are the rocky Great Basin and deserts such as the Chihuahua and Mojave. The Sierra Nevada and Cascade mountain ranges run close to the Pacific coast, both ranges reaching altitudes higher than 14,000 feet (4,300 m). The lowest and highest points in the contiguous United States are in the state of California, and only about 84 miles (135 km) apart. At an elevation of 20,310 feet (6,190.5 m), Alaska’s Denali (Mount McKinley) is the highest peak in the country and North America. Active volcanoes are common throughout Alaska’s Alexander and Aleutian Islands, and Hawaii consists of volcanic islands. The supervolcano underlying Yellowstone National Park in the Rockies is the continent’s largest volcanic feature.
The U.S. ecology is megadiverse: about 17,000 species of vascular plants occur in the contiguous United States and Alaska, and over 1,800 species of flowering plants are found in Hawaii, few of which occur on the mainland. The United States is home to 428 mammal species, 784 bird species, 311 reptile species, and 295 amphibian species. About 91,000 insect species have been described. The bald eagle is both the national bird and national animal of the United States, and is an enduring symbol of the country itself.
There are 58 national parks and hundreds of other federally managed parks, forests, and wilderness areas. Altogether, the government owns about 28% of the country’s land area.