Availability is the property that systems and data are accessible and usable when needed.
Availability focuses on keeping services usable for legitimate users. It is supported by redundancy, capacity planning, incident response, and protections against denial-of-service attacks.
The property of being accessible and usable upon demand.
In cybersecurity, applies to assets such as information or information systems.
The property of a system or a system resource being accessible, or usable or operational upon demand, by an authorized system entity, according to performance specifications for the system; i.e., a system is available if it provides services according to the system design whenever users request them. (See: critical, denial of service. Compare: precedence, reliability, survivability.)
"The property of being accessible and usable upon demand by an authorized entity." [I7498-2]
"Timely, reliable access to data and information services for authorized users." [C4009]
Deprecated Definition: IDOCs SHOULD NOT use the term with definition 3; the definition mixes "availability" with "reliability", which is a different property. (See: reliability.)
Tutorial: Availability requirements can be specified by quantitative metrics, but sometimes are stated qualitatively, such as in the following: