adj.1. describing or relating to a feature by which the failure of a component or subsystem of a machine or work system results in automatic shutdown or switch to a safer mode of operation. This fail-safe design reduces the chance that an operator will be exposed to hazards related to the malfunction or that machinery will be damaged. Fail-safe designs do not, however, reduce the likelihood of an accident or failure. See also exclusion design; prevention design. 2. more generally, having no or an extremely low probability of failure.