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, D.C. metropolitan area. As a result, the county is often referred to in the region simply as "Arlington" or "Arlington, Virginia". In 2013, the county's population was estimated to be 227,146, and would be the fourth-largest Virginian city if it were incorporated as such.The land that became Arlington was originally donated by Virginia to the United States government to form part of the new federal capital district. On February 27, 1801, a year after moving from the temporary National Capital at Philadelphia occupying the Federal City (also sometimes known then as "Washington City") in the District of Columbia, the United States Congress organized the area as a subdivision of the District named Alexandria County. In 1846, Congress returned the land southwest of the Potomac River donated by Virginia due to issues involving Congressional representation and the abolition of slavery. The General Assembly of Virginia changed the county's name to Arlington in 1920 to avoid confusion with the adjacent City of Alexandria.