Arlington, county (1990 pop. 170,936), N Va., across the Potomac River from Washington, D.C. Arlington is a residential and commercial suburb of Washington. Within its boundaries are Arlington National Cemetery; Arlington House, The Robert E. Lee Memorial; the Pentagon; the U.S. Marine Corps and Air Force memorials; Marymount Univ.; a campus of George Mason Univ.; Ronald Reagan National Airport; and Crystal City, a large multiuse development near the Potamac. Arlington has many federal facilities and major office developments as well as some industry, including printing and publishing, shipbuilding, and light manufacturing; defense contractors, information technology firms, and government research offices are clustered there. Originally Alexandria Co., Arlington was ceded to the federal government by Virginia in 1789 and was part of the District of Columbia until 1846, when it was returned to Virginia. In 1920 its name was changed to Arlington, after Arlington House, Robert E. Lee's home.