Definition of terms

Replacement migration: rate or migration that maintains the general or working-age population at a given level.

Mega city: A mega city is generally defined as a metropolitan area with a total population in excess of 10 million people.

the example of mega cities are

1. Tokyo, (33,600,000)
2. Seoul, South Korea (23,400,000)
3. Mexico City (22,400,000)
4. New York City , (21,961,994)
5. Mumbai (Bombay), India (21,600,000)
6. Delhi, India (21,500,000)
7. São Paulo, Brazil (20,600,000)
8. Los Angeles, USA (18,000,000)
9. Shanghai, China (17,500,000)

Threshold population: The minimum number of customers required by a service to succeed