1. The degree to which a system, subsystem, or equipment is operable and in a committable state at the start of a mission, when the mission is called for at an unknown, i.e., a random, time. Note 1: The conditions determining operability and committability must be specified. Note 2: Expressed mathematically, availability is 1 minus the unavailability. 2. The ratio of (a) the total time a functional unit is capable of being used during a given interval to (b) the length of the interval. Note 1: An example of availability is 100/168 if the unit is capable of being used for 100 hours in a week. Note 2: Typical availability objectives are specified in decimal fractions, such as 0. 9998. 3. Timely, reliable access to data and information services for authorized users. 4. The prevention of denial of service. 5. The property of being accessible and useable upon demand by an authorized entity. 6. The prevention of the unauthorized withholding of information or resources. 7. The property of an object being accessible and usable upon demand by an authorized subject.