[feyl-seyf] 
adjective, noun, verb, -safed, -saf⋅ing.| 1. | Electronics. pertaining to or noting a mechanism built into a system, as in an early warning system or a nuclear reactor, for insuring safety should the system fail to operate properly. |
| 2. | equipped with a secondary system that insures continued operation even if the primary system fails. |
| 3. | (sometimes initial capital letter ) of, pertaining to, or designating a system of coded military controls in which bombers dispatched to a prearranged point as part of a standard operating procedure cannot advance farther without direct orders from a designated authority and cannot have the nuclear warheads they carry armed until they have passed their prearranged point. |
| 4. | guaranteed to work; totally reliable: a fail-safe recipe for a cheese soufflé. |
| 5. | (sometimes initial capital letter) the point beyond which the bombers cannot go without specific instruction; the fail-safe point. |
| 6. | something designed to work or function automatically to prevent breakdown of a mechanism, system, or the like. |
| 7. | to make fail-safe. |