Source: U.S. Bureau of the Census: Metropolitan Areas. New areas usually are added annually; definitions of existing areas updated only after each decennial census, in years ending in 3. http://www.census.gov/population/metro/
The general concept of a metropolitan area is that of a large population nucleus, together with adjacent communities having a high degree of social and economic integration with that core. Metropolitan areas comprise one or more entire counties, except in New England, where cities and towns are the basic geographic units.
The Office of Management and Budget (OMB) defines metropolitan areas for purposes of collecting, tabulating, and publishing federal data. Metropolitan area definitions result from applying published standards to Census Bureau data.
A metropolitan areas identified as a consolidated metropolitan statistical area (CMSA) has a population of one million or more and also has separate component areas (PMSAs - primary metropolitan statistical areas) meeting statistical criteria and supported by local opinion.