A key factor in Britain's vote to leave the EU was concern over the difficulty of limiting migration into Britain due to the EU's open border policies.

Migration

The movement of people from one place to another with the intentions of settling, permanently or temporarily, at a new location.

Open Border Policies

pertain to government regulations that allow for free movement of people across national borders, reducing or eliminating immigration restrictions.

EU

The European Union, a political and economic union of 27 European countries that participate in a single market allowing free movement of goods, services, capital, and people.