Washington, D.C., formally the District of Columbia and commonly referred to as Washington, "the District", or simply D.C., is the capital of the United States. On July 16, 1790, the United States Congress approved the creation of a federal district to become the national capital as permitted by the U.S. Constitution. The District is therefore not a part of any U.S. state. It was formed from land along the Potomac River donated by the states of Maryland and Virginia; however, the Virginia portion was returned by Congress in 1846.

A new capital city named after George Washington was founded in 1791 to the east of the preexisting port of Georgetown. Congress consolidated the City of Washington, Georgetown, and the remaining unincorporated area within the District under a single municipal government in 1871. The city shares its name with the U.S. state of Washington, located on the country's Pacific coast.

Washington, D.C., had an estimated population of 617,996 in 2011. The city was the 24th most populous place in the United States as of 2010. Commuters from the surrounding Maryland and Virginia suburbs raise the city's population to over one million during the workweek. The Washington Metropolitan Area, of which the District is a part, has a population of nearly 5.6 million, the seventh-largest metropolitan area in the country.

The centers of all three branches of the federal government of the United States are located in the District, as are many of the nation's monuments and museums. Washington, D.C., hosts 176 foreign embassies as well as the headquarters of the World Bank, the International Monetary Fund (IMF), the Organization of American States (OAS), the Inter-American Development Bank, and the Pan American Health Organization (PAHO). The headquarters of many other institutions such as trade unions, non-profit organizations, lobbying groups, and professional associations are also located in the city.

The District is governed by a locally elected mayor and 13-member city council. However, the United States Congress has supreme authority over the city and may overturn local laws. Residents therefore have less self-governance than residents of the U.S. states. The District has a non-voting, at-large Congressional delegate, but no senators. D.C. residents could not vote in presidential elections until the ratification of the Twenty-third Amendment to the United States Constitution in 1961.