The Common Travel Area is an open borders area comprising the United Kingdom, Ireland, the Isle of Man and the Channel Islands. The British Overseas Territories are not included. Based on agreements that are not legally binding, the internal borders of the CTA are subject to minimal controls and can normally be traversed by British and Irish citizens with minimal identity documents. The maintenance of the CTA involves co-operation on immigration matters between the British and Irish authorities.

An open border is a border that enables free movement of people between jurisdictions with no restrictions on movement and is lacking substantive border control. A border may be an open border due to intentional legislation allowing free movement of people across the border, or a border may be an open border due to a lack of legal controls, a lack of adequate enforcement or adequate supervision of the border. An example of the former is the Schengen Agreement between most members of the European Economic Area. An example of the latter has been the border between Bangladesh and India, which is becoming controlled. The term "open borders" applies only to the flow of people, not the flow of goods and services, and only to borders between political jurisdictions, not to mere boundaries of privately owned property.