Definition: The functional and physical characteristics required to exist at a common boundary or connection between persons, between systems, or between persons and systems. A system external to the system being analyzed that provides a common boundary or service that is necessary for the other system to perform its mission in an undergraded mode, e.g., a system that supplies power, cooling, heating, air services, or input signals.

Citation: DAU : DAU Glossary of Defense Acquisition Acronyms and Terms