Blast Radius
The set of things that break, or become reachable, when one component fails or is compromised.
A single term that connects reliability and security, because the question is the same in both: if this thing goes wrong, how far does the damage reach?
Reducing blast radius is what cells, availability zones, per-tenant isolation, separate accounts and subscriptions, network segmentation and least-privilege roles are all actually for. None of them prevent failure or compromise; they bound its extent.
The useful design exercise is to name the largest blast radius in the system out loud — usually a shared database, a shared identity, a global configuration change, or a deployment pipeline with production credentials — and then decide deliberately whether to shrink it or accept it.