Arlington County is a county in the Commonwealth of Virginia. It is coterminous with the U.S. Census Bureau-census-designated place of Arlington, which is the second-largest principal city of the Washington metropolitan area. As a result, the county is often referred to in the region simply as "Arlington" or "Arlington, Virginia".
Due to the county's proximity to downtown Washington, D.C., Arlington is headquarters to many departments and agencies of the federal government of the United States, including the Department of Defense (DoD) at the Pentagon, Drug Enforcement Administration, Transportation Security Administration (TSA), and Defense Advanced Research Projects Agency (DARPA). It is also home to Ronald Reagan Washington National Airport. The many federal agencies, government contractors, and service industries contribute to Arlington's stable economy. Arlington is also the location of national memorials and museums, including Arlington National Cemetery, the Pentagon Memorial, the Marine Corps War Memorial, and the United States Air Force Memorial.