Safety integrity is the probability of a safety-related system satisfactorily performing the required safety functions under all the stated conditions within a stated period of time. (EN 61508 Part 4)
The required safety integrity of protection systems must be defined so that the frequency of failures of the safety-related systems is low enough to prevent the frequency of the hazardous event from exceeding the frequency required to achieve the tolerable risk level.
Safety integrity levels are used to classify safety functions with regard to their safety integrity.
[... more information on classification into safety integrity levels]
