The Beltway is a term for I-495, the circular route around DC and its inner suburbs. It does not refer to DC as a city since it encloses a huge area far beyond the borders of Washington, but "inside the Beltway" refers to all the people, companies, and agencies that are part of the federal government and its mindset.
A few years ago “The Beltway” was known as “The Ring Around the Congo.”