The United States of America (USA), commonly known as the United States (U.S.) or America, is a federal republic composed of 50 states, a federal district, five major self-governing territories, and various possessions. At 3.8 million square miles (9.8 million km2) and with over 325 million people, the United States is the world's third- or fourth-largest country by total area and the third-most populous. The capital is Washington, D.C., and the largest city by population is New York City.
Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. Europe is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, and the Mediterranean Sea to the south. It comprises the westernmost part of Eurasia. Europe is generally considered as separated from Asia by the watershed divides of the Ural and Caucasus Mountains, the Ural River.
Asia is Earth's largest and most populous continent, located primarily in the Eastern and Northern Hemispheres. It shares the continental landmass of Eurasia with the continent of Europe and the continental landmass of Afro-Eurasia with both Europe and Africa. Asia covers an area of 44,579,000 square kilometres (17,212,000 sq mi), about 30% of Earth's total land area and 8.7% of the Earth's total surface area.
Australia officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania and numerous smaller islands. It is the largest country in Oceania and the world's sixth-largest country by total area. The neighbouring countries are Papua New Guinea, Indonesia and East Timor to the north; the Solomon Islands and Vanuatu to the north-east; and New Zealand to the south-east. Australia's capital is Canberra, and its largest urban area is Sydney.
Africa is the world's second largest and second most-populous continent (the first being Asia in both categories). At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area and 20.4% of its total land area.With 1.2 billion people as of 2016, it accounts for about 16% of the world's human population.