Reliability Definitions Reliability is a time dependent characteristic. It can only be determined after an elapsed time but can be predicted at any time. It is the probability that a product or service will operate properly for a specified period of time (design life)under the design operating conditions without failure. CLGO 8+ - 8 Reliability Definitions  Reliability is a time dependent characteristic.  It can only be determined after an elapsed time but can be predicted at any time.  It is the probability that a product or service will operate properly for a specified period of time (design life) under the design operating conditions without failure