SIL stands for Safety Integrity Level. A SIL is a measure of safety system performance, or probability of failure on demand (PFD) for a SIF or SIS. There are four discrete integrity levels associated with SIL.
The higher the SIL level, the lower the probability of failure on demand for the safety system and the better the system performance. It is important to also note that as the SIL level increases, typically the cost and complexity of the system also increase.
A SIL level applies to an entire system. Individual products or components do not have SIL ratings. SIL levels are used when implementing a SIF that must reduce an existing intolerable process risk level to a tolerable risk range.