Washington, D.C., is not a state; it is a federal district. Since it functions as the United States' capital, D.C. is considered an administratively separate entity from any of the states in the Union so that no state can be said to have an unfair advantage in political representation.
Its status as a district rather than a state means that the residents of Washington, D.C., are disadvantaged in some ways. D.C. does not elect members of the U.S. Congress, so residents of the district are not represented in the federal government. Furthermore, until 1973, the area was completely governed by Congress and did not have its own mayor or city council.