A megalopolis (sometimes called a megapolis) is defined as an extensive metropolitan area or a long chain of roughly continuous metropolitan areas. The term was used by Lewis Mumford in his 1938 book, The Culture of Cities, which described it as the first stage in urban overdevelopment and social decline. Later, it was used by Jean Gottmann in 1957, to describe the huge metropolitan area along the Eastern seaboard of the U.S. extending from Boston, Massachusetts through New York City; Philadelphia, Pennsylvania; and ending in Washington, D.C. According to Gottmann, it resulted from changes in work and social habits.

The concept was later extended to include the following regions: BosWash (Boston–Washington), ChiPitts (Chicago to Pittsburgh), Quebec City–Windsor Corridor, SanSan, and Bajalta California. A megalopolis is also frequently a megacity, megapolitan area, or a metropolitan area with a total population in excess of 10 million people. Megalopolis is used in urban studies as a term to link the metropolitan Combined Statistical Areas of Boston–Worcester–Manchester, MA–RI–NH; Springfield, MA–Holyoke, MA, Hartford–West Hartford–Willimantic, CT; New York–Newark–Bridgeport, NY–NJ–CT–PA; Philadelphia–Camden–Vineland, PA–NJ–DE–MD; and Washington, D.C.–Baltimore–Northern Virginia, DC–MD–VA–WV.

The Pittsburgh–Chicago Corridor is an urban studies term that describes the area running through the Rust Belt from the Mid-Atlantic States to the Western Great Lakes region, although great spans of agricultural land and woodlots separates the urban areas. Within this megalopolis, the Steel City Corridor describes the area connecting Cleveland to Pittsburgh via Youngstown and Warren, Ohio, and Sharon–Farrell–New Castle, Pennsylvania. Historically, these areas are known as the Steel Valleys (along the Mahoning and Shenango rivers).

Modern interlinked ground transportation corridors, such as rail and highway, often aid in the development of megalopolises. Using these commuter passageways to travel throughout the megalopolis is called megaloping.

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