urbanization
migration
It is when people move from one urban area or city to another.
Lemmings.
The first mass migration was the American Army that invaded Mexico and started the Mexican American War.
Great Puritan Migration
The great migration
Migration from the latin Migratio
California Gold Rush
Migration from the latin Migratio
To move from city to city, within your country (state).
b mass migration
it can be called migration yet it is also referred to as urbanization.