A million city is, yes you guessed it, a city with one million (or more) inhabitants. These are the largest cities on the planet:

1. Tokyo (Japan) 27.2 million
2. Seoul (South Korea) 20.5 million
3. Mexico City (Mexico), 20.45 million
4. New York (USA) 19.75 million
5. Mumbai, India, 19.2 million

source: R.L. Forstall, R.P. Greene, and J.B. Pick, “Which are the largest? Why published populations for major world urban areas vary so greatly”, City Futures Conference, (University of Illinois at Chicago, July 2004) – Table 5 (p.34) 

You can find out more about the largest cities in the world (including predictions for the future) here.