Migration the ability to meet current needs without reducing the ability to meet future needs 2. immigration the leaving of one's homeland to settle in a new place 3. migration a situation in which the birth rate is not sufficient to replace the existing population 4. overpopulation the movement of people to a new country 5. sustainable development the increase in the population of a city 6. underpopulation a situation in which resources are being used up at a faster rate than they can be replenished 7. unsustainable development a situation in which the number of people exceeds the available natural resources in an area 8. urbanization the movement of people from one place to another