A megacity is defined by the United Nations as a metropolitan area with a total population of more than 10 million people. Some definitions also set a minimum level for population density (at least 2,000 persons/square km).
A megacity can be a single metropolitan area or two or more metropolitan areas, which have grown to such an extent, that they now form one urban area.
The terms megapolis or megalopolis are sometimes used synonymously with megacity. The terms metacity or hypercity are also sometimes used to describe cities with more than 20 million people.