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". In 2015, the county's population was estimated at 229,164, which would make it the fourth-largest city in the Commonwealth if it were incorporated as such.