Limes

Līmes is a modern term used primarily for the Germanic border defence or delimiting system of Ancient Rome marking the borders of the Roman Empire, but it was not used by the Romans for that purpose. The term has been extended to refer to the frontier defences in other parts of the empire, such as in the east and in Africa.

The līmes is often associated with Roman forts, but the concept could apply to any adjoining area the Romans exercised loose control with military forces.

Source: Wikipedia