The United States of America (USA), commonly referred to as the United States (US), America, or simply the States, is a federal republic consisting of 50 states, 16 territories, a federal district, and various overseas extraterritorial jurisdictions. The 48 contiguous states and the federal district of Washington, D.C., are in central North America between Canada and Mexico. The state of Alaska is the northwestern part of North America and the state of Hawaii is an archipelago in the mid-Pacific. The country also has five populated and nine unpopulated territories in the Pacific and the Caribbean. At 3.79 million square miles (9.83 million km2) in total and with around 316 million people, the United States is the fourth-largest country by total area and third largest by population. 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 is also extremely diverse, and it is home to a wide variety of wildlife.
Paleo-indians migrated from Asia to what is now the US mainland around 15,000 years ago, with European colonization beginning in the 16th century. The United States emerged from 13 British colonies located along the Atlantic seaboard. Disputes between Great Britain and these colonies led to the American Revolution. On July 4, 1776, delegates from the 13 colonies unanimously issued the Declaration of Independence. The ensuing war ended in 1783 with the recognition of independence of the United States from the Kingdom of Great Britain, and was the first successful war of independence against a European colonial empire. The current Constitution was adopted on September 17, 1787. The first 10 amendments, collectively named the Bill of Rights, were ratified in 1791 and guarantee many fundamental civil rights and freedoms.
The Arctic Ocean, located in the Northern Hemisphere and mostly in the Arctic north polar region, is the smallest and shallowest of the world's five major oceanic divisions. The International Hydrographic Organization (IHO) recognizes it as an ocean, although some oceanographers call it the Arctic Mediterranean Sea or simply the Arctic Sea, classifying it a mediterranean sea or an estuary of the Atlantic Ocean. Alternatively, the Arctic Ocean can be seen as the northernmost part of the all-encompassing World Ocean.
Almost completely surrounded by Eurasia and North America, the Arctic Ocean is partly covered by sea ice throughout the year (and almost completely in winter). The Arctic Ocean's surface temperature and salinity vary seasonally as the ice cover melts and freezes; its salinity is the lowest on average of the five major oceans, due to low evaporation, heavy fresh water inflow from rivers and streams, and limited connection and outflow to surrounding oceanic waters with higher salinities. The summer shrinking of the ice has been quoted at 50%. The US National Snow and Ice Data Center (NSIDC) uses satellite data to provide a daily record of Arctic sea ice cover and the rate of melting compared to an average period and specific past years.
West Coast or Pacific Coast are terms for the westernmost coastal states of the United States, usually California, Oregon, and Washington. More specifically, the term refers to an area defined on the east by the Cascade Range, Sierra Nevada and Mojave Desert and on the west by the Pacific Ocean. The U.S. Census groups the five states of California, Oregon, Washington, Alaska, and Hawaii together as the Pacific States division.
As of the 2010 Census, the estimated population of the Census Bureau's Pacific Region was approximately 47.8 million (56.9 million if Nevada and Arizona are included) - about 15.3% (18.2% with Nevada and Arizona) of US population.
The Alaska Statehood Act was signed by President Dwight D. Eisenhower on July 7, 1958, allowing Alaska to become the 49th U.S. state on January 3, 1959.
From 1867 to 1884 Alaska was considered to be a military district of the United States of America under the control of the federal government. Alaskans had sought statehood since as early as the 1920s though this vision was not realized until the decade after World War II.
A state of the United States of America is one of the 50 constituent political entities that shares its sovereignty with the United States federal government. Because of the shared sovereignty between each U.S. state and the U.S. federal government, an American is a citizen of both the federal republic and of his or her state of domicile. State citizenship and residency are flexible and no government approval is required to move between states, except for persons covered by certain types of court orders (e.g., paroled convicts and children of divorced spouses who are sharing custody).
States are divided into counties or county-equivalents, which may be assigned some local governmental authority but are not sovereign. County or county-equivalent structure varies widely by state. Kentucky, Massachusetts, Pennsylvania, and Virginia use the official title of Commonwealth rather than State.
The Western United States, commonly referred to as the American West or simply the West," traditionally refers to the region comprising the westernmost states of the United States. Because the U.S. expanded westward after its founding, the meaning of the West has evolved over time. Prior to about 1800, the crest of the Appalachian Mountains was seen as the western frontier. Since then, the frontier moved further west and the Mississippi River was referenced as the easternmost possible boundary of the West.
The West mostly comprises arid to semi-arid plateaus and plains and forested mountains.