Availability

Availability of an information service 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."

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