"A megalopolis (sometimes improperly called a megapolis) or megaregion is typically defined as a chain of roughly adjacent metropolitan areas. The term was used by Patrick Geddes in his 1915 book Cities in Evolution,[1] by Oswald Spengler in his 1918 book, The Decline of the West, and Lewis Mumford in his 1938 book, The Culture of Cities, which described it as the first stage in urban overdevelopment and social decline."
vs megacity: http://en.wikipedia.org/wiki/Megacity
"A megacity is usually defined as a metropolitan area with a total population in excess of ten million people.[1] A megacity can be a single metropolitan area or two or more metropolitan areas that converge. The terms conurbation, metropolis and metroplex are also applied to the latter."
NYC alone is a megacity. The US northeast is a megalopolis.