For the purposes of this blog I've chosen a set of popular definitions for terms that are likely to appear frequently. Of course many of these definitions vary, but these are the most common and straightforward.
Regional Terms
Metro Area: Area consisting of a highly populated city core and its surrounding suburban (and exurban) counties. Included counties must be at most two counties removed from the principal city or have at least 100,000 residents.
Baltimore Metropolitan Area: Baltimore City (MD), Anne Arundel County (MD), Baltimore County (MD), Carroll County (MD), Harford County (MD), Howard County (MD), Queen Anne's County (MD)
Washington Metropolitan Area: Washington, D.C., Calvert County (MD), Charles County (MD), Frederick County (MD), Montgomery County (MD), Prince George's County (MD), Howard County (MD), Arlington County (VA), Fairfax County (VA), Loudoun County (VA), Prince William County (VA), Stafford County (VA), Alexandria and other independent cities in Northern Virginia
Baltimore-Washington Metropolitan Area: Baltimore Metro Area + Washington Metro Area
Mid-Atlantic: New York, New Jersey, Pennsylvania, Delaware, Maryland, Washington D.C., [Northern Virginia]*, [West Virginia]*
New England: Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, Connecticut
Northeast: New England + Mid-Atlantic
Locality Definitions
Major Town: 25,000-50,000 residents
Small City: 50,000-100,000
Small City: 50,000-100,000
Mid-Sized City: 100,000-500,000 residents
Large City: 500,000+ residents
Low Density: <5,000 residents/sq. mi.
High Density: >5,000 residents/sq. mi.