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, founded on July 16, 1790. The City of Washington was originally a separate municipality within the Territory of Columbia until an act of Congress in 1871 effectively merged the City and the Territory into a single entity called the District of Columbia. It is for this reason that the city, while legally named the District of Columbia, is known as Washington, D.C.