5. Glossary

Arbre de défaillance, arbre de panne; fault tree

Logic diagram showing sub-entity failures, external events or their combinations, which have caused a predefined undesired event.

Availability, operating status; up state

A condition characterized by the ability to function as required.

State of unavailability, state of failure; down state

A condition characterized by inability to function as required.

Instantaneous availability, A(t)

Probability of being available at a given time.

Instantaneous unavailability, U(t)

Probability of being unavailable at a given time.

Average availability, ${}^{}$