Washington, D.C., officially the District of Columbia, serves as the capital city and federal district of the United States. Situated on the Potomac River, it shares land borders with Maryland to its north and east, while facing Virginia across the water. The city anchors the southern end of the Northeast megalopolis and functions as the seat of the U.S. federal government, hosting the White House, the U.S. Capitol, and the Supreme Court Building among other federal headquarters.

The U.S. Constitution in 1789 called for a federal district under exclusive congressional jurisdiction, leading to the Residence Act of July 16, 1790, which approved the capital along the Potomac River. Named after George Washington and Columbia, the nation's personification, the district was formed from land donated by Maryland and Virginia. Congress held its first session there in November 1800, and the District of Columbia Organic Act of 1801 officially organized the territory under federal control.

Designed in 1791 by Pierre Charles L'Enfant, the city is split into quadrants meeting at the Capitol Building, containing 131 neighborhoods. It hosts numerous national monuments and museums on or around the National Mall, including the Jefferson Memorial, Lincoln Memorial, and Washington Monument. The city also houses 177 foreign embassies and global headquarters for organizations like the World Bank and International Monetary Fund, alongside a lobbying hub centered on K Street.

As an important world political capital, Washington, D.C., is among the country's top tourist destinations, attracting millions of visitors annually. The city offers a cosmopolitan atmosphere with shopping, dining, and nightlife comparable to a world-class metropolis. Beyond the National Mall, tourists explore vibrant districts and nearby suburbs like Arlington and Alexandria, which offer ethnic dining and attractions such as Arlington Cemetery and the Iwo Jima Memorial.

The city's population was 689,545 as of the 2020 census, with daytime commuters increasing this figure to over one million. The Washington metropolitan area, including parts of Maryland, Virginia, and West Virginia, had a 2023 population of 6.3 million residents. Governed by a locally elected mayor and council since 1973, the district retains unique political status, with residents electing non-voting delegates and presidential electors under the Twenty-third Amendment.