Probability of Failure on Demand average- This is the probability that a system will fail dangerously, and not be able to perform its safety function when required. PFD can be determined as an average probability or maximum probability over a time period. IEC 61508/61511 and ISA 84.01 use PFDavg as the system metric upon which the SIL is defined.
Also Process Flow Diagram. A diagram of the basic process equipment usually accompanied by a heat and material balance. Typically prepared early in an engineering project, it is usually part of the input to a HAZOP or other hazard identification process.