A city and/or metropolitan area with a very high population or average density. A megacity is generally considered to be an urban agglomeration with a population of at least 10 million, though the United Nations defines it as a metro area that is home to at least 5 million people living in an area of consistent urban-level density. Most of the world's megacities are in the developing world — particularly sub-Saharan Africa and South and East Asia — which is rapidly urbanizing to the same high percentage that is seen in the United States, Latin America, and Western Europe. However, these cities are generally built with little in the way of construction regulation or public infrastructure. By the year 2030, it is estimated that more than 60 percent of the world's population will be urban.
Today the largest megacity is by far Greater Tokyo, with includes the nearby cities of Yokohama and Kawasaki and is home to nearly 35 million people; however, its current population growth is practically stagnant. The other nine largest are:
* Mexico City, D.F. (22,350,000)
* New York (22,150,000)
* Seoul, South Korea (22,050,000)
* São Paulo, Brazil (20,000,000)
* Mumbai (Bombay), India (19,400,000)
* Delhi, India (19,000,000)
* Los Angeles (17,750,000)
* Tehran, Iran (15,000,000)
* Jakarta, Indonesia (16,850,000)
* Osaka, Japan (16,750,000)
Smaller megacities include Bogotá, Colombia; Lagos, Nigeria; Manila, the Philippines; Shanghai, China; Rio de Janeiro, Brazil; Beijing, China; Karachi, Pakistan; London; Paris; Istanbul, Turkey; Chicago; Cairo, Egypt; Dhaka, Bangladesh; Ruhr Valley, Germany; and Buenos Aires, Argentina.