The Americas, or America, are lands in the Western Hemisphere that are also known as the New World. Comprising the continents of North America and South America, along with their associated islands, they cover 8.3% of the Earth's total surface area (28.4% of its land area). The topography is dominated by the American Cordillera, a long chain of mountains that run the length of the west coast. The flatter eastern side of the Americas is dominated by large river basins, such as the Amazon, Mississippi, and La Plata. Extending 14,000 km (8,699 mi) in a north-south orientation, the climate and ecology varies strongly across the Americas, from arctic tundra of Northern Canada, Greenland, and Alaska, to the tropical rain forests in Central America and South America. When the continents joined 3 million years ago, the Great American Interchange resulted in many species being spread across the Americas, such as the cougar, porcupine, opossums, armadillos and hummingbirds.
Humans first settled the Americas from Asia between 40,000 BCE and 15,000 BCE. A second migration of Na-Dene speakers followed later from Asia. The subsequent migration of the Inuit into the neoarctic around 3500 BCE completed what is generally regarded as the settlement by the Indigenous peoples of the Americas. The first European discovery of and settlement in the Americas was by the Norse explorer Leif Ericson. However the colonization never became permanent and was later abandoned. The voyages of Christopher Columbus from 1492 to 1502 resulted in permanent contact with European (and subsequently, other Old World) powers, which led to the Columbian exchange. Diseases introduced from Europe and Africa devastated the Indigenous peoples, and the European powers colonised the Americas. Mass emigration from Europe, including large numbers of indentured servants, and forced immigration of African slaves largely replaced the Indigenous Peoples. Beginning with the American Revolution in 1776 and Haitian Revolution in 1791, the European powers began to decolonise the Americas. Currently, almost all of the population of the Americas resides in independent countries; however, the legacy of the colonisation and settlement by Europeans is that the Americas share many common cultural traits, most notably the predominant adherence to Christianity and use of Indo-European languages; primarily Spanish, English, and Portuguese. More than 900 million people live in the Americas (about 13.5% of the human population), the most populous countries being the United States, Brazil, and Mexico, the most populous cities being Mexico City, São Paulo, and New York City.
The earliest known use of the name America dates to April 25, 1507, where it was applied to what is now known as South America. It appears on a small globe map with twelve time zones, together with the largest wall map made to date, both created by the German cartographer Martin Waldseemüller in Saint-Dié-des-Vosges in France. These were the first maps to show the Americas as a land mass separate from Asia. An accompanying book, Cosmographiae Introductio, anonymous but apparently written by Waldseemüller's collaborator Matthias Ringmann, states, "I do not see what right any one would have to object to calling this part [that is, the South American mainland], after Americus who discovered it and who is a man of intelligence, Amerigen, that is, the Land of Americus, or America: since both Europa and Asia got their names from women". Americus Vespucius is the Latinized version of the Florentine explorer Amerigo Vespucci's name, and America is the feminine form of Americus. Amerigen is explained as Amerigo plus gen, the accusative case of the Greek word for 'earth', and meaning 'land of Amerigo'. (See etymology.) Amerigo itself is an Italian form of the medieval Latin Emericus (see also Saint Emeric of Hungary), which through the German form Heinrich (in English, Henry) derived from the Germanic name Haimirich.
Vespucci was apparently unaware of the use of his name to refer to the new landmass, as Waldseemüller's maps did not reach Spain until a few years after his death. Ringmann may have been misled into crediting Vespucci by the widely published Soderini Letter, a sensationalized version of one of Vespucci's actual letters reporting on the mapping of the South American coast, which glamorized his discoveries and implied that he had recognized that South America was a continent separate from Asia; in fact, it is not known what Vespucci believed on this count, and he may have died believing what Columbus had, that they had reached the East Indies in Asia rather than a new continent. Spain officially refused to accept the name America for two centuries, saying that Columbus should get credit, and Waldseemüller's later maps, after Ringmann's death, did not include it; however, usage was established when Gerardus Mercator applied the name to the entire New World in his 1538 world map. Acceptance may have been aided by the "natural poetic counterpart" that the name America made with Asia, Africa, and Europa.
In modern English, North and South America are generally considered separate continents, and taken together are called the Americas in the plural, parallel to similar situations such as the Carolinas. When conceived as a unitary continent, the form is generally the continent of America in the singular. However, without a clarifying context, singular America almost invariably refers to the United States of America.