A metropolitan area or metro is a region of a densely populated urban agglomeration and its surrounding territories sharing industries, commercial areas, transport network, infrastructures and housing. A metro area usually comprises multiple principal cities, jurisdictions and municipalities: neighborhoods, townships, boroughs, cities, towns, exurbs, suburbs, counties, districts, as well as even states and nations like the euro districts. As social, economic and political institutions have changed, metropolitan areas have become key economic and political regions.
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Metropolitan areas typically include satellite cities, towns and intervening rural areas that are socioeconomically tied to the principal cities or urban core, often measured by commuting patterns. Metropolitan areas are sometimes anchored by one central city such as the Paris metropolitan area (Paris) or Mumbai Metropolitan Region (Mumbai). In other cases metropolitan areas contain multiple centers of equal or close to equal importance especially in the United States, for example the Dallas–Fort Worth metropolitan area has 8 principal cities. The Islamabad–Rawalpindi metropolitan area (Islamabad and Rawalpindi), the Rhine-Ruhr in Germany and the Randstad in the Netherlands are other examples.
In the United States, the concept of metropolitan statistical areas has gained prominence. The area of the Greater Washington metropolitan area is an example of statistically grouping independent cities and county areas from various states to form a larger city because of proximity, history and recent urban convergence. Metropolitan areas may themselves be part of a greater megalopolis. For urban centres located outside metropolitan areas that generate a similar attraction at smaller scale for a region, the concept of a regiopolis and respectively regiopolitan area or regio was introduced by German professors in 2006. In the United States, the term micropolitan statistical area is used.