A metropolitan area (metropolis) is a region consisting of a densely populated urban core and its less populated surrounding territories, sharing industry, infrastructure and housing. A metro area usually comprises multiple jurisdictions and municipalities, neighborhoods, townships, boroughs, cities, towns, exurbs, suburbs, counties, districts, states and even nations. As social, economic and political institutions have changed, metropolitan areas have become key economic and political regions. Metropolitan areas include satellite cities, towns and intervening rural areas that are socioeconomically tied to the urban core, typically measured by commuting patterns. 

At first, the ‘metropolitan district’ was used in 1910 in the Census of Population carrying a central city of 50,000 or more population and all adjacent minor civil divisions having a population density of 150 or more persons per square mile. By 1940, the concept of the metropolitan district had lost favour because the possibilities of correlating local data with district data were limited. 

Besides this, the metropolitan district did not truly represent social and economic integration with the central city. Therefore, a new areal unit, the standard metropolitan area, came into vogue in 1949. It was defined as made up of counties instead of minor civil divisions. It was succeeded by the ‘Standard Metropolitan Statistical Area’ (SMSA).It is defined below as it existed in 1970. 

A contiguous county will be included in an SMSA if: 

(a) At least 75% of the resident labour force in the county is in non-agricultural labour sector 

(b) At least 30% of the employed workers living in the county work in the central county or counties of the area. 

A contiguous county which does not meet the foregoing requirements will be included in an SMSA if at least 75% of the resident labour force is non-agricultural and meets two of the following additional criteria of metropolitan character and one of the following criteria of integration. 

(i) Criteria of Metropolitan Character 

(a) At least 25% of the population is urban. 

(b) The county had an increase of at least 15% in total population during the period covered by the most recent two censuses. 

(c) The county has a population density of at least 50 persons per square mile. 

(ii) Criteria of Integration 

(a) A least 15% of the employed workers living in the county work in the central county or counties of the area, or 

(b) The number of people working in the county who live in the central county or counties of the area is equal to at least 15 per cent of the employed workers living in the county, or 

(c) The sum of the number of workers commuting to and from the central county or counties is equal to at least 20% of the employed workers living in the county.

As stated above it seems that metropolis is primarily a demographic concept. But this may be emphasized once again that metropolis is much more than an agglomeration. It possesses distinct character and functions which are not likely to be found either in agglomeration or in conurbation. In metropolitan cities each function has benefited from the conditions which brought about the other functions and has found reasons for developing there itself. 

There the powers of attraction make them bigger and bigger, and consequently enormity of their size increases their power of attraction. This is happening in Mumbai which has grown tremendously during 1901-1991 by 1000 times. Mumbai has simultaneously an international port, one of the biggest commercial marts, an important industrial node of the country and a cosmopolitan centre. After analyzing the conditions of this development, it may be observed that out of 300 cities with population over one million in the world (1991), more than 50% are sea ports. In reality, ports possess immense potentials to concentrate functions, because, on the one hand, they are by definition commercial places, and on the other, they attract manufacturing industries by the materials of all sorts which they receive from various parts of the country. 

Even in the days of the ancient Mediterranean civilizations, and also, during the colonial times, the great commercial connections were then maritime links with the metropolis. The eastern and western sides of the Atlantic in Europe and America therefore, have great ports. Metropolitan cities acquire a sort of permanence and remain evergreen over time.

Metropolitan Cities in India 

The Constitution (74th Amendment) Act, 1992 defines a metropolitan area in India as, an area having a population of ten lakhs or more, comprised in one or more districts and consisting of two or more municipalities or panchayats or other contiguous areas, specified by the Governor by public notification to be a Metropolitan area. 

In India, the Census Commission defines a metropolitan city as one having a population of over four million. Delhi, Mumbai, Kolkata, Chennai, Hyderabad, Bangalore, Ahmedabad, Pune, Surat and Nashik are those Indian cities that have over 4 million people. For these million plus cities the Census definition of an urban agglomeration requires that it should be a continuous urban spread constituting a town and its adjoining urban growths or two or more physically contiguous towns together with adjoining outgrowths. 

There are 53 urban agglomerations in India with a population of 1 million or more as of 2011 against 35 in 2001. As per the preliminary results of the Census 2011, released by the Registrar General of India, Greater Mumbai with a population of 18,414,288 continues to be India’s biggest city, followed by Delhi – 16,314,838 and Kolkata- 14,112,536. These three cities are India’s mega-cities with 10 million plus population. But, when we consider Urban Agglomeration as an extended city comprising built up area of central core and any suburbs linked by continuous urban area, we have a change at the top. Delhi NCR, with the inclusion of Gurgaon, Faridabad, Noida and Ghaziabad becomes the No.1 urban agglomeration with a population of 21,753,486, ahead of 20,748,395. 

As of 2011 census of India, there are 46 metropolitan cities in India and the top ten are, Mumbai, Delhi, Kolkata, Chennai, Hyderabad, Bengaluru, Ahmedabad, Pune, Surat and Visakhapatnam.