Washington, D.C., (pronounced /ˈwɑʃɪŋtən diˈsi/) is the capital of the United States. It is coterminous with the District of Columbia (abbreviated as "D.C."). The city and the district are located on the banks of the Potomac River and bordered by the states of Virginia (to the west) and Maryland (to the north, east and south). The city was planned and developed in the late 18th century to serve as the permanent national capital; the federal district was formed to keep the national capital distinct from the states.

The city was named after George Washington, the first President of the United States. The district's name, "Columbia", is an early poetic name for the United States and a reference to Christopher Columbus, an early explorer of the Americas. The city is commonly referred to as Washington, The District, or simply D.C. In the 19th century, it was called the Federal City or Washington City. Its population is about 588,292. The Washington Metropolitan Area is the eighth-largest in the United States with more than five million residents, and the Baltimore-Washington Metropolitan Area has a population exceeding eight million. If Washington, D.C., were a state, it would rank last in area (behind Rhode Island), second to last in population (ahead of Wyoming), first in population density, and 35th in gross state product.

The centers of all three branches of the U.S. government are located in the District. Also situated in the city are the headquarters for the World Bank, the International Monetary Fund, the Organization of American States, the Inter-American Development Bank, and other national and international institutions, including labor unions and professional associations. Washington is a frequent location for political demonstrations and protests, large and small, particularly on the National Mall. A center of American history and culture, Washington is a popular destination for tourists, the site of numerous national landmarks and monuments, the world's largest museum complex (the Smithsonian Institution), galleries, universities, cathedrals, performing arts centers and institutions, and native music scenes. The District also includes substantial areas of surprisingly wild natural habitat, particularly along the Potomac and Anacostia rivers, as well as most parts of Rock Creek Park and Theodore Roosevelt Island (located in the Potomac River).

The District of Columbia and the city of Washington are governed by a single municipal government and for most practical purposes, are considered to be the same entity. This has not always been the case: until 1871, when Georgetown ceased to be a separate city, there were multiple jurisdictions within the District. Although there is a municipal government and a Mayor, Congress has the supreme authority over the city and district, which results in citizens having less self-governance than residents of the states. The District has a non-voting at-large Congressional representative. In the financial year 2004, federal tax collections were $16.9 billion while federal spending in the District was $37.6 billion.