The United States of America (also referred to as the United States, the U.S., the USA, or America /əˈmɛrɪkə/) is a federal constitutional republic comprising fifty states and a federal district. The country is situated mostly in central North America, where its forty-eight contiguous states and Washington, D.C., the capital district, lie between the Pacific and Atlantic Oceans, bordered by Canada to the north and Mexico to the south. The state of Alaska is in the northwest of the continent, with Canada to the east and Russia to the west across the Bering Strait. The state of Hawaii is an archipelago in the mid-Pacific. The country also possesses several territories in the Caribbean and Pacific.

At 3.79 million square miles (9.83 million km²) and with over 309 million people, the United States is the third or fourth largest country by total area, and the third largest both by land area and population. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries.^[7] The U.S. economy is the world's largest national economy, with an estimated 2009 GDP of $14.3 trillion (a quarter of nominal global GDP and a fifth of global GDP at purchasing power parity).^[4][8]

Indigenous peoples of Asian origin have inhabited what is now the mainland United States for many thousands of years. This Native American population was greatly reduced by disease and warfare after European contact. The United States was founded by thirteen British colonies located along the Atlantic seaboard. On July 4, 1776, they issued the Declaration of Independence, which proclaimed their right to self-determination and their establishment of a cooperative union. The rebellious states defeated the British Empire in the American Revolution, the first successful colonial war of independence.^[9] The current United States Constitution was adopted on September 17, 1787; its ratification the following year made the states part of a single republic with a strong central government. The Bill of Rights, comprising ten constitutional amendments guaranteeing many fundamental civil rights and freedoms, was ratified in 1791.

In the 19th century, the United States acquired land from France, Spain, the United Kingdom, Mexico, and Russia, and annexed the Republic of Texas and the Republic of Hawaii. Disputes between the agrarian South and industrial North over states' rights and the expansion of the institution of slavery provoked the American Civil War of the 1860s. The North's victory prevented a permanent split of the country and led to the end of legal slavery in the United States. By the 1870s, the national economy was the world's largest.^[10] The Spanish–American War and World War I confirmed the country's status as a military power. It emerged from World War II as the first country with nuclear weapons and a permanent member of the United Nations Security Council. The end of the Cold War and the dissolution of the Soviet Union left the United States as the sole superpower. The country accounts for two-fifths of global military spending and is a leading economic, political, and cultural force in the world.^[11]

The United States has been a leader in scientific research and technological innovation since the late 19th century. In 1876, Alexander Graham Bell was awarded the first U.S. patent for the telephone. Thomas Edison's laboratory developed the phonograph, the first long-lasting light bulb, and the first viable movie camera. Nikola Tesla pioneered alternating current, the AC motor, and radio. In the early 20th century, the automobile companies of Ransom E. Olds and Henry Ford promoted the assembly line. The Wright brothers, in 1903, made the first sustained and controlled heavier-than-air powered flight.^[101]

The rise of Nazism in the 1930s led many European scientists, including Albert Einstein and Enrico Fermi, to immigrate to the United States. During World War II, the Manhattan Project developed nuclear weapons, ushering in the Atomic Age. The Space Race produced rapid advances in rocketry, materials science, and computers. The United States largely developed the ARPANET and its successor, the Internet. Today, the bulk of research and development funding, 64%, comes from the private sector.^[102] The United States leads the world in scientific research papers and impact factor.^[103] Americans possess high levels of technological consumer goods,^[104] and almost half of U.S. households have broadband Internet access.^[105] The country is the primary developer and grower of genetically modified food; more than half of the world's land planted with biotech crops is in the United States.^[106]

Everyday personal transportation in America is dominated by the automobile. As of 2003, there were 759 automobiles per 1,000 Americans, compared to 472 per 1,000 inhabitants of the European Union the following year.^[108] About 40% of personal vehicles are vans, SUVs, or light trucks.^[109] The average American adult (accounting for all drivers and nondrivers) spends 55 minutes driving every day, traveling 29 miles (47 km).^[110]

The civil airline industry is entirely privately owned, while most major airports are publicly owned. The four largest airlines in the world by passengers carried are American; Southwest Airlines is number one.^[111] Of the world's thirty busiest passenger airports, sixteen are in the United States. It is also home to the busiest airport in the world, Hartsfield-Jackson Atlanta International Airport.^[112] While transport of goods by rail is extensive, relatively few people use rail to travel, within or between cities.^[113] Mass transit accounts for 9% of total U.S. work trips, compared to 38.8% in Europe.^[114] Bicycle usage is minimal, well below European levels.

^{From Wikipedia}