PFDavg:
Average Probability of Failure on Demand. This is the probability that a LOW DEMAND mode function or equipment item will fail dangerously and not be able to perform its safety function when required. 

PFDavg of a SIF assumes no more than one demand per year, and must be calculated with a practical proof test interval. In the context of IEC 61511 the PFDavg of an entire safety instrumented function should be calculated to verify that it meets the criteria of a specific SIL target.