Greater Pittsburgh is a populous region centered around Pittsburgh, Pennsylvania, the region's largest city and economic hub. The region encompasses Pittsburgh's urban core county, Allegheny, and six adjacent Pennsylvania counties: Armstrong, Beaver, Butler, Fayette, Washington, and Westmoreland in Western Pennsylvania, which constitutes the Pittsburgh, PA Metropolitan Statistical Area MSA as defined by the U.S. Census Bureau. As of the 2020 census, the Greater Pittsburgh region had a population of over 2.37 million people. Roughly one-fifth of the entire population of Pennsylvania resides within the region. The core city, Pittsburgh, has a population of 302,971, making it the second-largest city in the state. Over half of the region's population resides within Allegheny County, which has a population of 1.24 million and is the second-largest county by population in the state. Definitions Garrett Nelson and Alasdair Rae's 2016 analysis of American commuter flows, "An Economic Geography of the United States: From Commutes to Megaregions", identified the Pittsburgh megaregion as a region encompassing the entirety or significant portions of 54 counties in Western Pennsylvania, Eastern Ohio, Northern West Virginia, and Western Maryland. By this definition, the informal regional boundaries are similar to historical interpretations where the region is defined as the central portion of the Allegheny Plateau to the west and north of the Allegheny Front and south of Lake Erie and Pennsylvania's Northern Tier. The hills and river valleys along the Upper Ohio River and its many eastern tributaries, including the Allegheny and Monongahela rivers have historically been the major centers of population of the region. According to this research the US can be divided into 'mega regions' based on the most extreme commuting patterns within a geographical area (commutes within 100 miles from the core city). By this liberal definition, the Pittsburgh mega region consists of twenty-eight Pennsylvania counties (Allegheny, Armstrong, Beaver, Bedford, Blair, Butler, Cambria, Cameron, Centre, Clarion, Clearfield, Crawford, Elk, Erie, Fayette, Forest, Greene, Huntingdon, Indiana, Jefferson, Lawrence, Mercer, Mifflin, Somerset, Venango, Warren, Washington, and Westmoreland), nineteen West Virginia counties (Barbour, Brooke, Doddridge, Grant, Hancock, Harrison, Lewis, Marion, Marshall, Mineral, Monongalia, Ohio, Preston, Randolph, Taylor, Tucker, Tyler, Upshur, and Wetzel), five Ohio counties (Belmont, Columbiana, Harrison, Jefferson, and Monroe), and two Maryland counties (Allegany and Garrett), and portions of Chautauqua, New York in and around Ripley. . The combined population of the megaregion was over 4.9 million in 2016. There are also several formal definitions of Greater Pittsburgh which