Definition from Wiktionary, the free dictionary
- A 64-mile Interstate freeway surrounding Washington, D.C..
- (mostly local usage) The expressway that surrounds another city.
- (US, politics) The US federal government and policy and lobbying organizations, located in Washington, D.C..
- Of or relating to the culture of Washington, D.C.; politicized.
1993 January 6, Mark Feeney, “Impeach me tender”, Boston Globe, page 28:
- Apparently wishing to go Beltway in a big way, the Gap reportedly solicited Clinton spokesman George Stephanopoulos and media adviser Mandy Grunwald to pose for ads, but were turned down.
1997, Kurt Finsterbusch, Annual Editions: Sociology, 97-98:
- In a typical indictment, one columnist recently called some piece of Washington policymaking "too secret, too expert, too Beltway."
- 2002 
- Your New Yorker article posed the question, "Can the president's education crusade survive Beltway politics?"
2003 September 21, Howard Fineman, “Dean: Not Just A New-School Kinda Guy”, Newsweek:
- And wouldn't you know, the real-life Dean, in a real-life debate, used a line the show's writers had proposed for him. Nothing more Beltway than that.