A expression of the degree to which a system, subsystem, or equipment is not operable and not in a committable state at the start of a mission, when the mission is called for at an unknown, i.e., random, time. Note 1: The conditions determining operability and committability must be specified. Note 2: Expressed mathematically, unavailability is 1 minus the availability. Note 3: Unavailability may also be expressed mathematically as the ratio of the total time a functional unit is not capable of being used during a given interval to the length of the interval, e. G. , if the unit is not capable of being used for 68 hours a week, the unavailability is 68/168.