in a states-transitions model, state belonging to a given class of states and which is distant from the
failure class of states by only one transition
Note 1 to entry: This is a mathematical concept in relationship with e.g. Markovian process or Petri nets models.
EXAMPLE The states of a safety system can be sorted out into two classes: class OK when the safety action is
available and KO when the safety action is inhibited. In this case a critical state with regards to the safety system
failure belongs to the class OK and only one failure (i.e. on event) is needed to have a transition to the class KO.
Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global Standards