<b>Safety Integrity Level</b> (SIL) is defined as a relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. In simple terms, SIL is a measurement of performance required for a Safety Instrumented Function (SIF).

The requirements for a given SIL are not consistent among all of the functional safety standards. In the European Functional Safety standards based on the IEC 61508 standard four SILs are defined, with SIL 4 being the most dependable and SIL 1 being the least. A SIL is determined based on a number of quantitative factors in combination with qualitative factors such as development process and safety life cycle management.

<a href=”http://en.wikipedia.org/wiki/Safety_Integrity_Level”>http://en.wikipedia.org/wiki/Safety_Integrity_Level</a>