A megalopolis, or megapolis, is defined as an extensive metropolitan area or a long chain of roughly continuous metropolitan areas in the United States and Canada. The term was first used in the United States by Jean Gottmann in 1957, to describe the huge urban area along the Eastern seaboard of the U.S. from Boston, Massachusetts to Washington, D.C. According to Gottmann, it resulted from changes in work and social habits. See also: BosWash, ChiPitts, 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-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 ideally 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.