Static Stability
The property that a system keeps working on its existing state when its control plane or dependencies are unavailable, rather than needing them to keep running.
The distinction is between the data plane (serving requests with what it already knows) and the control plane (making changes: provisioning, configuration, scaling, failover). A statically stable system's data plane does not require the control plane to be healthy.
Concretely: an instance keeps serving with its current configuration if the config service is down; a load balancer keeps routing to its current targets if it cannot reach the registry; a cached authorisation decision remains valid briefly rather than failing closed on every request.
The counter-example is the design that requires a call to the control plane on the request path, which is how a control-plane incident becomes a total outage. It is also why failover plans that require provisioning new capacity during a regional event are fragile — the control plane is exactly what is under stress. Pre-provisioned standby capacity is statically stable; scale-on- failover is not.
The design question: what does this system need to be working, right now, to serve the request it already knows how to serve? Every item on that list is an availability dependency.