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. The signing of the Residence Act on July 16, 1790, approved the creation of a capital district located along the Potomac River on the country's East Coast. As permitted by the U.S. Constitution, the District is under the exclusive jurisdiction of the Congress and is therefore not a part of any U.S. state.

The states of Maryland and Virginia each donated land to form the federal district, which included the preexisting settlements of Georgetown and Alexandria. Named in honor of George Washington, the City of Washington was founded in 1791 to serve as the new national capital. Congress returned the land originally ceded by Virginia in 1846 and created a single municipal government for the remaining portion of the District in 1871.

Washington, D.C., had an estimated population of 632,323 in 2012, the 24th most populous place in the United States. Commuters from the surrounding Maryland and Virginia suburbs raise the city's population to more than one million during the workweek. The Washington Metropolitan Area, of which the District is a part, has a population of 5.7 million, the seventh-largest metropolitan area in the country. The centers of all three branches of the federal government of the United States are in the District, including the Congress, President, and Supreme Court. Washington is home to many national monuments and museums, which are primarily situated on or around the National Mall. The city hosts 176 foreign embassies as well as the headquarters of many international organizations, trade unions, non-profit organizations, lobbying groups, and professional associations.

A locally elected mayor and 13-member council have governed the District since 1973; however, the Congress maintains supreme authority over the city and may overturn local laws. The District has a non-voting, at-large Congressional delegate, but no senators. The Twenty-third Amendment to the United States Constitution, ratified in 1961, grants the District three electoral votes in presidential elections.