A metropolitan area is a large population center that consists of several cities or towns clustered together with one or more large cities serving as its hub or hubs.
A conurbation is an urban area comprising a number of cities or towns whose built-up zones of influence have merged physically through population growth and expansion to form one contiguous urban cluster. A metropolitan area usually combines a conurbation proper (the contiguous built-up area) with peripheral zones not themselves necessarily urban in character but closely bound to the conurbation by employment or commerce.
If several metropolitan areas are located in succession, metropolitan areas are sometimes grouped together as as megalopolis. A megalopolis consists of several interconnected cities (and their suburbs), between which people commute, and which are so close together that suburbs can claim to be suburbs of more than one city. This concept was first proposed by the French geographer Jean Gottmann in his book Megalopolis, who studied the northeastern United States. One famous example is the BosWash megalopolis consisting of New York City, Boston, Baltimore, Washington, and Philadelphia and vicinity. Other megalopolis are Tokyo/Osaka, the industrialized region in Germany and the Low Countries, and London/Canterbury/several other cities in England. Fictional mega-cities feature in much dystopian science fiction, with cities such as the Sprawl and Mega-City One. In the comic 2000 A.D, the fictional Megacity One is a megalopolis of 800 million people across the east coast of the United States, policed by Judge Dredd.
Megacity is a general term for cities together with their suburbs or recognized metropolitan area usually with a total population in excess of 10 million people. Whereas the term city includes importance, density and legal status of a place, the term megacity concentrates on size only.
In 1950 New York was the only such area; there are currently (2002) twenty, with twelve of those areas having exceeded 10 million since 1990. This has happened as the entire world population moves towards the high (75-85%) urbanization levels of North America and Western Europe. It is not clear that any city exclusive of its suburbs exceeds 10 million.
As well as all these cities experiencing some growth, by 2015 there could be a further six megacities. However the expansion of megalopolis is probably a greater trend, such as the previously mentioned Tokyo-Osaka, or Baltimore-Washington or Rio de Janeiro-Sao Paulo.
In Canada, megacity refers to the results of having merged the suburbs of an urban region into one large municipality. Cities so merged include Winnipeg, Manitoba (this merger antedates the term, and was called "Unicity" at the time); Halifax, Nova Scotia; Toronto, Ontario; Ottawa, Ontario; Hamilton, Ontario; Montreal, Quebec; Gatineau, Quebec; Longueuil, Quebec; Quebec City, Quebec; and Saguenay, Quebec.