The United States of America, commonly referred to as the United States, America, or simply the States, is a federal republic consisting of 50 states and a federal district. 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 in total and with around 317 million people, the United States is the fourth-largest country by total area and third largest by population.

The United States is the world's third or fourth largest nation by total area, ranking behind Russia and Canada and just above or below China. The ranking varies depending on how two territories disputed by China and India are counted and how the total size of the United States is measured: calculations range from 3,676,486 square miles to 3,717,813 square miles to 3,794,101 square miles. Measured by only land area, the United States is third in size behind Russia and China, just ahead of Canada.

The United States, with its large size and geographic variety, includes most climate types. To the east of the 100th meridian, the climate ranges from humid continental in the north to humid subtropical in the south. The southern tip of Florida is tropical, as is Hawaii. The Great Plains west of the 100th meridian are semi-arid. Much of the Western mountains are alpine. The climate is arid in the Great Basin, desert in the Southwest, Mediterranean in coastal California, and oceanic in coastal Oregon and Washington and southern Alaska.

English is the de facto national language. Although there is no official language at the federal level, some laws such as U.S. naturalization requirements standardize English. In 2010, about 230 million, or 80% of the population aged five years and older, spoke only English at home. Spanish, spoken by 12% of the population at home, is the second most common language and the most widely taught second language.

The First Amendment of the U.S. Constitution guarantees the free exercise of religion and forbids Congress from passing laws respecting its establishment. Christianity is by far the most common religion practiced in the U.S., but other religions are followed, too.