What is the term for the moving of a population out of an area?

Emigration. If it's a forced emigration, it's called expatriation. The dispersion of people from their native land is also known as a diaspora (with a lower-case D). The people who have been dispersed can also be referred to as a diaspora. If it's because of a natural disaster it also can be called "evacuation" (to a safer place.) * When a population moves from one area to another, such as people moving from the city to the suburbs it is referred to as migration, the same is applicable to persons who follow seasonal work such as migrant agriculture laborers. And of course it is applied to the animal and bird species that follow seasonal migration.