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 located on the east bank of the Potomac River, which forms its southwestern border with Virginia, and borders Maryland to its north and east. The city was named for George Washington, a Founding Father, commanding general of the Continental Army in the American Revolutionary War, and the first president of the United States, and the district is named for Columbia, the female personification of the nation.

Washington, D.C. represents the southern point of the Northeast megalopolis, one of the nation's largest and most influential cultural, political, and economic regions that runs along its northeast coast from Boston in the north to Washington, D.C. in the south and also includes New York City, Philadelphia, and Baltimore. As the seat of the U.S. federal government and several international organizations, the city is an important world political capital. It is one of the most visited cities in the U.S. with over 20 million annual visitors as of 2016.

The U.S. constitution provides for a federal district under the exclusive jurisdiction of the U.S. Congress. Washington, D.C., is not a part of any U.S. 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 Congress held its first session there in 1800. In 1801, the territory, formerly part of Maryland and Virginia and including the settlements of Georgetown and Alexandria, was officially recognized as the federal district. In 1846, Congress returned the land originally ceded by Virginia, including the city of Alexandria; in 1871, it created a single municipal government for the remaining portion of the district. There have been several unsuccessful efforts to make the city into a state since the 1880s, though a statehood bill passed the House of Representatives in 2021.

The city is divided into quadrants, which are centered around the Capitol and include 131 neighborhoods. As of the 2020 census, the city had a population of 689,545, making it the 23rd-most populous city in the U.S., third-most populous city in the Southeast behind Jacksonville and Charlotte, and third-most populous city in the Mid-Atlantic after New York City and Philadelphia. 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 sixth-largest metropolitan area with a 2020 population of 6.3 million residents; and over 54 million people live within 250 mi (400 km) of the city.

The city is the home to each of the three branches of the U.S. federal government, Congress (legislative), the President (executive), and the Supreme Court (judicial), along with the governmental buildings that house most of the federal government, 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, primarily located on or around the National Mall, including the Jefferson Memorial, the Lincoln Memorial, and the Washington Monument. The city hosts 177 foreign embassies and the headquarters of the World Bank, the International Monetary Fund, the Organization of American States, and other international organizations. Many of the nation's largest industry associations, non-profit organizations, and think tanks are based in the city, including AARP, the American Red Cross, Atlantic Council, Brookings Institution, National Geographic Society, The Heritage Foundation, Wilson Center, and others.

A locally elected mayor and 13-member council have governed the district since 1973. Congress maintains supreme authority over the city, however, and is empowered to overturn local laws. Washington, D.C., residents are, on a federal level, politically disenfranchised since the city's residents do not have voting representation in Congress, although the city's residents elect a single at-large congressional delegate to the U.S. House of Representatives who has no vote. District voters choose three presidential electors in accordance with the Twenty-third Amendment, ratified in 1961. Washington, D.C. has been a member state of the Unrepresented Nations and Peoples Organization since 2015.