A physical dependency is a non-digital requirement that a service needs to keep operating, such as water flow, cooling, power, ventilation, or filtration. Even if software is healthy, the service can still fail when the supporting physical process is blocked, degraded, or removed.
This matters in cyber security because availability problems are not always caused by malware or hacking. An attacker may exploit a physical dependency by disrupting cooling, damaging intake systems, or forcing a safety shutdown, while defenders use the same concept to harden critical infrastructure. Good resilience planning maps these dependencies, adds redundancy, and monitors environmental signals so operators can detect a problem before it becomes an outage. In practice, a physical dependency is part of the attack surface whenever digital services rely on real-world machinery.