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SynAc
Term

Availability

Availability is the property that systems and data are accessible and usable when needed.

Senses

Accessible when required

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.

References
  • NIST CSRC GlossaryJan 05, 2026
    NIST CSRC Glossary — Availability
    https://csrc.nist.gov/glossary/term/availability
    NIST states most site information is public information and may be distributed or copied, except material marked as copyrighted; attribution requested. Verify per-document markings before quoting.
    Source: NIST CSRC Glossary (csrc.nist.gov).
Sense 2

The property of being accessible and usable upon demand.

In cybersecurity, applies to assets such as information or information systems.

References
  • NICCS (CISA) Cybersecurity VocabularyJan 06, 2026
    NICCS glossary export (CSV)
    https://niccs.cisa.gov/rest/vocab/export-csv
    NICCS is a CISA (DHS) program. Individual glossary entries include a "From" attribution (e.g., CNSSI 4009, NIST SPs, NICE Framework). Treat "From" values as upstream provenance and verify before quoting large portions of text.
    Source: NICCS (CISA) Cybersecurity Vocabulary (niccs.cisa.gov).
1 (I)

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.)

References
  • IETF RFC 4949 (Internet Security Glossary)Jan 06, 2026
    RFC 4949 — Internet Security Glossary (Version 2)
    https://www.rfc-editor.org/rfc/rfc4949.txt
    RFC 4949 is published by the IETF Trust and marked as "Distribution of this memo is unlimited". Verify IETF Trust copyright/licensing terms for reuse.
    Source: IETF RFC 4949 (rfc-editor.org).
2 (O)

"The property of being accessible and usable upon demand by an authorized entity." [I7498-2]

References
  • IETF RFC 4949 (Internet Security Glossary)Jan 06, 2026
    RFC 4949 — Internet Security Glossary (Version 2)
    https://www.rfc-editor.org/rfc/rfc4949.txt
    RFC 4949 is published by the IETF Trust and marked as "Distribution of this memo is unlimited". Verify IETF Trust copyright/licensing terms for reuse.
    Source: IETF RFC 4949 (rfc-editor.org).
3 (D)

"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:

  • "Flexible tolerance for delay" may mean that brief system outages do not endanger mission accomplishment, but extended outages may endanger the mission.
  • "Minimum tolerance for delay" may mean that mission accomplishment requires the system to provide requested services in a short time.
References
  • IETF RFC 4949 (Internet Security Glossary)Jan 06, 2026
    RFC 4949 — Internet Security Glossary (Version 2)
    https://www.rfc-editor.org/rfc/rfc4949.txt
    RFC 4949 is published by the IETF Trust and marked as "Distribution of this memo is unlimited". Verify IETF Trust copyright/licensing terms for reuse.
    Source: IETF RFC 4949 (rfc-editor.org).