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A metropolitan area, sometimes referred to as a metro area or commuter belt, is a region consisting of a densely populated
For urban centres outside metropolitan areas, that generate a similar attraction at smaller scale for their region, the concept of the
A metropolitan area combines an
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 contains multiple urban agglomerations not connected by continuous development. In defining a metropolitan area, it is sufficient that a city or cities form a nucleus with which other areas have a high degree of integration.
See also the many lists of metropolitan areas itemized at § Lists of metropolitan areas.