Definition
Availability
Availability is the property that authorised users can access and use a resource when needed:
The requirement is relative to specified operating conditions and response-time limits. A resource is not available merely because it exists: it must be reachable and usable for its intended purpose.
System Availability
System availability concerns the operation of a system: authorised users can obtain the required service promptly.
Data Availability
Data availability concerns access to data: authorised users can retrieve and use the required information.