Arlington County is a county in the Commonwealth of Virginia coterminous with the U.S. Census Bureau's census-designated place of Arlington, which is the second-largest principal city of the Washington metropolitan area in the United States. As a result, the county is often referred to in the region simply as "Arlington" or "Arlington, Virginia". In 2016, the county's population was estimated at 230,050, making it the sixth-largest county in Virginia, or the fourth-largest city if it were incorporated as such. It is the highest-income county in the U.S. by median family income, and has the highest concentration of singles in the region.The county is situated in Northern Virginia on the southwestern bank of the Potomac River directly across from Washington, D.C., of which it was once a part. With a land area of 26 square miles, Arlington is the geographically smallest self-governing county in the U.S.