adj. Said of a strategy that handles errors in a way that ensures the system will go into a nondangerous mode if the software or hardware crashes. It is often thought that fail safe means the system cannot fail. That’s impossible because everything physical can break. Instead, a fail-safe system dies in a safe mode: the nuclear power plant that shuts down instead of melting down, the ABS brakes that turn control over to a backup mechanical hydraulic system, and so on.