What do you mean by the "Northeast"?

The American Northeast is the urban aggregation extending from the northern reaches of metropolitan Boston down to the Virginia suburbs of Washington DC.  It is urban and suburban from end-to-end, and is one of the great megalopolises of the world.  Its population density is very close to 1,000 people per square mile, which is far in excess of 10,000 people a square mile in the most urbanized sections of the large cities of the Northeast.  Compare this to an average population density of 99 people per square mile for the United States overall.

Sometimes this area is called the Northeast Corridor after the name of the rail lines extending between these big cities. Even more oddly it is sometimes called the Eastern Seaboard.  I am not sure what a “seaboard” is, but it apparently does not include the coastal plain of the southern US states, meaning it really should be the Northeastern Seaboard.  Also, the concept of “the Northeast” does not include the entirety of the states of the northeast, in my humble opinion.  Any city on the Great Lakes is really part of the Midwest, sharing that region’s former glory and current challenges as the (former) source of natural resources such as coal, ore and oil; and as the (former) center of heavy industry.  Thus, western Pennsylvania including the former steel capital Pittsburgh and nearby cities and towns, and western and upstate New York including chemicals capital Rochester, manufacturing capital Buffalo and nearby cities and towns, are not really part of the Northeast.  They have more in common with places like Cleveland (the birthplace of the oil industry and the Rockefeller oil monopoly), Detroit (the birthplace of the American automotive industry), Milwaukee (birthplace of modern farm machinery) and the many smaller (post-) industrial cities that line the Great Lakes.  And other sections of this collection of states running from Maine to Virginia along the Atlantic coast are better characterized as Appalachia, especially upstate New Hampshire, the rugged Adirondacks and much of central New York State and Pennsylvania, as well as the Maryland panhandle and western Virginia.