Includes a city or cities whose population is specified as that of the central city together with the county or counties in which it is located.  The Metro also includes contiguous or additional counties when the economic and social relationships between the central and additional counties meet specific criteria. Nielsen Metros generally correspond to the Metropolitan Statistical Areas (MSAs) defined by the U.S. Government’s Office of Management and Budget.

Source:
http://www.nielsenmedia.com/glossary/
http://www.arbitron.com/downloads/terms_brochure.pdf