Washington is one word. DC is simply the name of the district it is in. If you want to include capital cities' territories/districts in their names, a whole heap of cities will qualify here. For example, Caracas is actually Caracas, DC (Distrito Capital) and Bogota is Bogota, DC (also Distritio Capital). More examples: Paris, Île-de-France; Amsterdam, North Holland; Brasilia, DF (Distrito Federal) - you get the idea...