What is the difference between a metropolitan area, a city, and a megalopolis?

A metropolitan area includes a core city as well as nearby communities, making it a larger settlement of people than just a city. A city is any urban settlement that is larger than a town (which generally has between 2,500 and 20,000 residents), and that is able to function on its own, with a government and other public services. A megalopolis is any multicity area with more than 10 million people. For example, as of 2010, in the United States, the third largest metropolitan area is the Chicago-Joliet-Naperville area, with 9.4 million residents, which also houses the third largest U.S. city, Chicago.