The United States of America (also referred to as the United States, the US, the USA, or America) is a federal constitutional republic comprising fifty states and a federal district. The land area of the contiguous United States is approximately 1,900 million acres (7,700,000 km 2). Alaska, separated from the contiguous United States by Canada, is the largest state at 365 million acres (1,480,000 km 2). Hawaii, occupying an archipelago in the central Pacific, southwest of North America, has just over 4 million acres (16,000 km 2). The United States is the world's third or fourth largest nation by total area (land and water), ranking behind Russia and Canada and just above or below China.
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 Mojave. The Sierra Nevada and Cascade mountain ranges run close to the Pacific coast. At 20,320 feet (6,194 m), Alaska's Mount McKinley is the tallest peak in the country and in North America. Active volcanoes are common throughout Alaska's Alexander and Aleutian Islands, and Hawaii consists of volcanic islands. The super volcano underlying Yellowstone National Park in the Rockies is the continent's largest volcanic feature.
The United States is a federal constitutional republic, in which the President of the United States (the head of state and head of government), Congress, and judiciary share powers reserved to the national government, and the federal government shares sovereignty with the state governments. The executive branch is headed by the President and is independent of the legislature. Legislative power is vested in the two chambers of Congress, the Senate and the House of Representatives. The judicial branch (or judiciary), composed of the Supreme Court and lower federal courts, exercises judicial power (or judiciary). The judiciary's function is to interpret the United States Constitution and federal laws and regulations. This includes resolving disputes between the executive and legislative branches. The federal government's layout is explained in the Constitution. Two parties, the Democratic Party and the Republican Party, have dominated American politics since the American Civil War, although other parties have also existed.
There are major differences between the political system of the United States and that of most other developed democracies. These include increased power of the upper house of the legislature, a wider scope of power held by the Supreme Court, the separation of powers between the legislature and the executive, and the dominance of only two main parties. The United States is one of the world's developed democracies where third parties have the least political influence.
The federal entity created by the Constitution is the dominant feature of the American governmental system. However, most people are also subject to a state government, and all are subject to various units of local government. The latter include counties, municipalities, and special districts.
This multiplicity of jurisdictions reflects the country's history. The federal government was created by the states, which as colonies were established separately and governed themselves independently of the others. Units of local government were created by the colonies to efficiently carry out various state functions. As the country expanded, it admitted new states modeled on the existing ones.