A metropolitan area, metro area or metro is a region consisting of a densely populated urban core and its less-populated surrounding territories, sharing industry, infrastructure, and housing. A metropolitan area usually comprises multiple jurisdictions and municipalities: neighborhoods, townships, cities, exurbs, counties, and even states. As social, economic and political institutions have changed, metropolitan areas have become key economic and political regions.
A metropolitan area combines an urban agglomeration (the contiguous, built-up area) with zones not necessarily urban in character, but closely bound to the center by employment or other commerce. These outlying zones are sometimes known as a commuter belt, and may extend well beyond the urban zone, to other political entities. For example, El Monte, California is considered part of the Los Angeles' metro area in the United States.
In practice, the parameters of metropolitan areas, in both official and unofficial usage, are not consistent. Sometimes they are little different from an urban area, and in other cases they cover broad regions that have little relation to a single urban settlement; comparative statistics for metropolitan area should take this into account. Population figures given for one metro area can vary by millions.
There has been no significant change in the basic concept of metropolitan areas since its adoption in 1950, although significant changes in geographic distributions have occurred since then, and more are expected. Because of the fluidity of the term "metropolitan statistical area," the term used colloquially is more often "metro service area," "metro area," or "MSA" taken to include not only a city, but also surrounding suburban, exurban and sometimes rural areas, all which it is presumed to influence.
A polycentric metropolitan area is one not connected by continuous development or conurbation, which requires urban contiguity. In defining a metropolitan area, it is sufficient that a city or cities constitute a nucleus with which other areas have a high degree of integration.