Here is what BBC wrote:
Mass urbanisation, the increasing concentration of populations into towns and cities, is one of the defining characteristics of the middle of the 20th Century the two biggest economies, the USA and Japan, had created a new urban phenomenon: the megacity. By 2025, seven of the world's top ten megacities will be in Asia.
|Cities with over 10 million inhabitants. Infographic originally appeared on BBC|
> This image of the earth at night shows where the population of the world is crowding together into major urban areas. By the middle of the 20th Century this urbanisation had led to a new phenomenon: the megacity — an urban area with over 10 million inhabitants (BBC).