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.