fail safe
1. Of a device, the capability to fail without detriment to other devices or danger to personnel.
2. Pertaining to the automatic protection of programs and/or processing systems to maintain safety when a hardware or software failure is detected in a system. [NIS]
3. [The] automatic protection of programs and/or processing systems when hardware or software failure is detected. [INFOSEC-99]
4. Pertaining to the structuring of a system such that either it cannot fail to accomplish its assigned mission regardless of environmental factors or that the probability of such failure is extremely low.