1. In a system , a shared boundary, i.e., the boundary between two subsystems or two devices. 2. A shared boundary between two functional units, defined by specific attributes, such as functional characteristics, common physical interconnection characteristics, and signal characteristics. 3. A point of communication between two or more processes, persons, or other physical entities. 4. A point of interconnection between user terminal equipment and commercial communications facilities. 5. To interconnect two or more entities at a common point or shared boundary. 6. A common boundary between independent systems or modules where interactions take place. 7. The point of interconnection between two distinct but adjacent communications systems having different functions. The interface in the communication-satellite service is that point where communications terminal equipment of the terrestrial common carriers or other authorized entities interconnects with the terminal equipment of the communication-satellite earth station complex. The interface in the communication-satellite service shall be located at the earth station site, or if this is impracticable, as close thereto as possible. 8. The shared boundary between two functional units, defined by functional characteristics and other characteristics, as appropriate.