What is a SIL?

A Safety Integrity Level (SIL) is a discrete level (one out of a possible four), each corresponding to a range of target likelihood of probability of failure on demand, for specifying the safety integrity requirements of the safety functions to be allocated to the E/E/PE safety-related systems, where SIL 4 has the highest level of safety integrity and SIL 1 has the lowest.

It is important to note that SIL is a characteristic of the full safety function rather than a logic system or individual component.