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Availability of an information service, in the context of information security means that authorized users can depend on being able to use it whenever it is needed. In U.S. law, a definition is "Ensuring timely and reliable access to and use of information." [1]

To specify availability in detail, however, can be quite complex. For example, if a service is offered only during normal business hours between 9 and 5, the service fails at 4, is repaired at 11, and is working the next morning, has it been unavailable for:

  • 1 hour, the period of outage during business hours
  • 7 hours, the actual period the service was unusable if someone attempted to use it
  • 17 hours, the perceived period of outage if someone saw it fail and left for the day, not knowing he could respect it in the morning


  1. 44 USC 3542