Basic Information Security and Privacy Glossary

Compiled by  John Ruero, Vice President ISSA-PH


The probability that a system or information is available at a given time. Availability may be expressed in general terms as the reliability of a system, or more specifically as the percentage of time a system is up and running. In the latter case, this is usually given as a series of “nines”, i.e. five nines means the system is up 99.999% of the time.

The triad of confidentiality, integrity, and availability are the core principles of information security. Most security efforts are focused on maintaining these.

For information to be useful, it must be available to authorized parties at the time they need it. Methods like backups, database replication, or redundant network carriers are commonly used to ensure information remains available.

Challenges to availability might come from hardware or software problems leading to a system outage. Or, availability might be affected by a malicious cause such as a denial-of-service (DoS) attack.