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Washington, D.C., formally the District of Columbia and commonly known as Washington or D.C., is the capital city and federal district of the United States. The city is on the Potomac River, across from Virginia, and shares land borders with Maryland to its north and east. It was named for George Washington, the first president of the United States. The district is named for Columbia, the female personification of the nation.

The U.S. Constitution in 1789 called for the creation of a federal district under the exclusive jurisdiction of the U.S. Congress. As such, Washington, D.C., is not part of any state, and is not one itself. The Residence Act, adopted on July 16, 1790, approved the creation of the capital district along the Potomac River. The city was founded in 1791, and the 6th Congress held the first session in the unfinished Capitol Building in 1800 after the capital moved from Philadelphia. In 1801, the District of Columbia, formerly part of Maryland and Virginia and including the existing settlements of Georgetown and Alexandria, was officially recognized as the federal district; initially, the city was a separate settlement within the larger district. In 1846, Congress returned the land originally ceded by Virginia, including the city of Alexandria. In 1871, it created a single municipality for the remaining portion of the district. There have been several unsuccessful efforts to make the district into a state since the 1880s; a statehood bill passed the House of Representatives in 2021 but was not adopted by the U.S. Senate.

Designed in 1791 by Pierre Charles L'Enfant, the city is divided into quadrants, which are centered around the Capitol Building and include 131 neighborhoods. As of the 2020 census, the city had a population of 689,545. Commuters from the city's Maryland and Virginia suburbs raise the city's daytime population to more than one million during the workweek. The Washington metropolitan area, which includes parts of Maryland, Virginia, and West Virginia, is the country's seventh-largest metropolitan area, with a 2023 population of 6.3 million residents. A locally elected mayor and 13-member council have governed the district since 1973, though Congress retains the power to overturn local laws. Washington, D.C., residents are, on the federal level, politically disenfranchised since the city's residents do not have voting representation in Congress; the city's residents elect a single non-voting congressional delegate to the U.S. House of Representatives. The city's voters choose three presidential electors in accordance with the Twenty-third Amendment.

Washington, D.C. anchors the southern end of the Northeast megalopolis. As the seat of the U.S. federal government, the city is an important world political capital. The city hosts the buildings that house federal government headquarters, including the White House, the Capitol, the Supreme Court Building, and multiple federal departments and agencies. The city is home to many national monuments and museums, located most prominently on or around the National Mall, including the Jefferson Memorial, the Lincoln Memorial, and the Washington Monument. It hosts 177 foreign embassies and serves as the headquarters for the World Bank, the International Monetary Fund, the Organization of American States, and other international organizations. Home to many of the nation's largest industry associations, non-profit organizations, and think tanks, D.C. is known as a lobbying hub, with K Street as the industry center. The city had 20.7 million domestic visitors and 1.2 million international visitors, ranking seventh among U.S. cities as of 2022.