A metropolitan area is a large population center consisting of a large metropolis and its adjacent zone of influence, or of more than one closely adjoining neighboring central cities and their zone of influence. One or more large cities may serve as its hub or hubs, and the metropolitan area is normally named after either the largest or most important central city within it.
There has been no significant change in the basic metropolitan area "concept" since its adoption in 1950 , though significant changes in geographic distributions have occurred since, and is expected to further evolve through time.  Because of the fluidity and evolution of the "term" metropolitan statistical areas, the colloquial reference by the general population and media to define an MSA is with a more familiar reference to "metro service area, metro area, metro, or MSA" and widely intimated to mean the aggregate geographic area inclusive of not only a well known city population, but also its inner city, suburban, exurban and sometimes rural surrounding populations, all of which are influenced by employment, transportation, and commerce of the more largely well known urban city.
A metropolitan area usually combines an agglomeration (the contiguous built-up area) with peripheral zones not themselves necessarily urban in character, but closely bound to the center by employment or commerce. These zones are also sometimes known as a commuter belt, and may extend well beyond the urban periphery depending on the definition used. It is mainly the area that is not part of the city but is connected to the city. For example, Pasadena, California would be added to Los Angeles' metro area. While it isn't the same city, it is connected, and Pasadena is also located in Los Angeles County.
The core cities in a polycentric metropolitan area need not be physically connected by continuous built-up development, distinguishing the concept from conurbation, which requires urban contiguity. In a metropolitan area, it is sufficient that central cities together constitute a large population nucleus with which other constituent parts have a high degree of integration.
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 the traditional concept of a city as a single urban settlement. Thus all metropolitan area figures should be treated as interpretations rather than as hard facts. Metro area population figures given by different sources for the same place can vary by millions, and there is a tendency for people to promote the highest figure available for their own "city". However the most ambitious metropolitan area population figures are often better seen as the population of a "metropolitan region" than of a "city".
The term metropolitan area is sometimes abbreviated to 'metro', for example in Metro Manila and Washington, DC Metro Area, which in the latter case should not be mistaken to metro rail system of the city. Although it can be compared in composition to many of the world's metropolitan areas, in France the term for the region around an urban core linked by commuting ties is an aire urbaine (officially translated as "urban area"). In Japan that would be toshiken (都市圏, lit. bloc of cities).
In Australia, Statistical Divisions (SDs) are defined by the Australian Bureau of Statistics as areas under the unifying influence of one or more major towns or cities. Each capital city forms its own Statistical Division, and the population of the SD is the most-often quoted figure for that city's population. Statistical Districts are defined as non-capital but predominantly urban areas. The statistical divisions that encompass the capital cities are commonly though unofficially called 'metropolitan areas'.
The European Union's statistical agency, Eurostat, has created a concept named Larger Urban Zone (LUZ). The LUZ represents an attempt at a harmonised definition of the metropolitan area, and the goal was to have an area from a significant share of the resident commute into the city, a concept known as the “functional urban region”.
In India, the Census Commission defines a metropolitan city as one having a population of over 40 lakh (4 million). Mumbai, Delhi, Chennai, Kolkata, Bengaluru, Hyderabad, Ahmedabad, Cochin, Pune are the eight cities that qualify. Residents of these cities are also entitled to a higher House rent allowance. The figure only applies to the city region and not the conurbation.
The Office of Management and Budget defines "Core Based Statistical Areas" used for statistics purposes among federal agencies. Each CBSA is based on a core urban area and is composed of the counties which comprise that core as well as any surrounding counties that are tightly socially or economically integrated with it. These areas are designated as either metropolitan or micropolitan statistical areas, based on population size; a "metro" area has an urban core of at least 50,000 residents, while a "micro" area has less than 50,000 but at least 10,000.
This concept was first proposed by the French geographer Jean Gottmann in his book Megalopolis, a study of the northeastern United States. One famous example is the Northeast megalopolis consisting of Boston, New York City, Philadelphia, Baltimore, Washington, and their vicinities.
The biggest one is the Taiheiyō Belt (the Pacific megalopolis) in Japan consisting of Tokyo, Shizuoka, Nagoya, Osaka, Okayama, Hiroshima, Fukuoka and vicinity. The main transportation such as Shinkansen and expressways is constructed along these cities. The population of this megalopolis is around 82.9 million.
Guangdong Province's Pearl River Delta is a huge megalopolis with a population of 48 million that extends from Hong Kong and Shenzhen to Guangzhou. Some projections assume that by 2030 up to 1 billion people will live in China's urban areas. Even rather conservative projections predict an urban population of up to 800 million people. In its most recent assessment, the UN Population Division estimated an urban population of 1 billion in 2050.
The megalopolises in Europe are the Milan metropolitan area (pop. 7.4 million) in Italy, Ruhr Area (pop. 5.3 million) in Germany, the Randstad (Knooppunt Arnhem-Nijmegen and Brabantse Stedenrij are counted with the Randstad) in the Netherlands (pop. 7.4 million), the Flemish Diamond in Belgium (pop. 5.5 million), Ile de France in France and the metropolitan area of London and Moscow, as well as several 'smaller' agglomerations, such as the Meuse-Rhine Euregion, the Ems-Dollart Euregion, the Lille-Kortrijk-Tournai Euregion and Metropoly of Upper Silesia in Poland (17 cities around Katowice with a total population of over 2 million). Together this megalopolis has an estimated population of around 50 million.
Africa's first megalopolis is situated in the urban portion of Gauteng Province in South Africa, comprising the conurbation of Johannesburg, and the metropolitan areas of Pretoria and the Vaal Triangle, otherwise known as the PWV.
It has been suggested that the whole of south-eastern, Midland and parts of northern England will evolve into a megalopolis dominated by London. Clearly when usage is stretched this far, it is remote from the traditional conception of a city.
Megacity is a general term for agglomerations or metropolitan areas which usually have a total population in excess of 10 million people. In Canada, "megacity" can also refer informally to the results of merging a central city with its suburbs to form one large municipality. A Canadian "megacity", however, is not necessarily an entirely urbanized area, as many cities so named have both rural and urban portions. It also doesn't need 10 million inhabitants to bear the designation. Moreover, Canadian "megacities" do not constitute large metropolitan areas in a global sense. For example, Toronto has a metropolitan population of 5.5 million but is part of a much larger metropolitan area home to over 8.1 million people.
Census population of a metro area is not the city population. However, it better demonstrates the population of the city. Los Angeles may only have a city population of near 4,000,000, but has two metropolitan area populations, depending on definition, 13 million in the core area and 18 million in the Combined statistical area.
A metropolitan area is a large area of developed land, which surrounds a city, and is often made up of many buildings. It can include smaller towns and villages which are now connected to the larger city as it has grown. The biggest such metropolitan area is that of Tokyo. 8.5 million people live in Tokyo itself, 36.5 million live in the metropolitan area.
Looking from the air down to a populated area, there will often appear one big "blob" of buildings and lights. The entire built-up area is the metropolitan area. The city is often just the part near the center of the "blob". However, it may not be in the center. Sometimes there is even more than one city inside the "blob".
So to put it simply, looking from the air again, the "blob" is the metropolitan area. The city is a smaller section of the "blob".