Central cities are urban counties with at least 50,000 inhabitants. A Metropolitan Statistical Area (MSA) contains a central city and the outlying suburban counties. An outlying county must have a high level of social and economic integration with the central city (or cities) to be considered part of the MSA. These outlying counties comprise the Suburban MSA.
Non-MSA urbanized areas are places outside the boundaries of MSAs with at least 2,500 inhabitants. A place can be an incorporated area, or a closely settled population center without corporate limits.
Areas not classified as urban are rural.
SOURCE: U.S. Department of Commerce, U.S. Census Bureau.