What does "safety margin" really mean?

The term "safety margin" has become a keyword when discussing about the safety of the nuclear plants, but there is still much confusion about the use of the term. In this paper the traditional concept of safety margin in nuclear engineering is described, and the need is expressed of extending the concept to out-of-design scenarios. A probabilistic definition of safety margin (PSM) is adopted for scalar safety Output at a scenario-specific level. The PSM is easy to generalize (to initiating events. Multiple Safety Outputs, analytical margins) and, combined with frequencies of initiators and accidents, makes up the plant risk. Both deterministic and probabilistic approaches to safety assessment found easy explanation in terms of PSM. The role of the probabilistic margins in the safety assessment of plant modifications is discussed.