In the United States a metropolitan statistical area (MSA) is a geographical region with a relatively high population density at its core and close economic ties throughout the area. Such regions are not legally incorporated as a city or town would be, nor are they legal administrative divisions like counties and states. As such the precise definition of any given metropolitan area can vary with the source. A typical metropolitan area is centered on a single large city that wields substantial influence over the region (e.g., Chicago, or Atlanta). However, some metropolitan areas contain more than one large city with no single municipality holding a substantially dominant position (e.g., Dallas–Fort Worth metroplex, Riverside–San Bernardino or Minneapolis–Saint Paul).
The U.S. Office of Management and Budget (OMB) defines a set of core based statistical areas (CBSAs) throughout the country. CBSAs are delineated on the basis of a central urban area or urban cluster—a contiguous area of relatively high population density. CBSAs are composed of counties and county-equivalents. The counties containing the core urban area are known as the central counties of the CBSA. Additional surrounding counties (known as outlying counties) can be included in the CBSA if these counties have strong social and economic ties to the central counties as measured by commuting and employment. Outlying counties are included in the CBSA if the employment interchange measure (total of in commuting and out commuting) is 25% or more, although these numbers are estimates, and exceptions are made. Note that some areas within these outlying counties may actually be rural in nature. CBSAs are subdivided into metropolitan statistical areas (MSAs) and micropolitan statistical areas based on the population of the core urban area. Under certain conditions, one or more CBSAs may be grouped together to form a larger statistical entity known as a combined statistical area (CSA). Other terms, such as standard metropolitan statistical area (SMSA) and primary metropolitan statistical area, were used in the past but were discontinued. In New England, because of the greater importance of towns over counties, similar areas are defined based on town units, known as New England city and town areas (NECTAs). U.S. Census statistics for metropolitan areas are reported based on these definitions.