System interdependence is the way separate services rely on one another to function as intended. In a modern environment, a customer portal may depend on identity services, databases, payment platforms, logging, and internal management tools. If one of those pieces fails, the others may still be running but the business service can become unusable.
This matters in cyber security because attackers often target the weakest or most connected component to create wider disruption. Ransomware, destructive malware, or even a misconfigured update can cascade through tightly linked systems and take down many services at once. Defenders use this concept when designing segmentation, redundancy, and fallback procedures, so they can isolate failure, restore critical components safely, and keep essential operations available even when one dependency is offline.