Availability definition

An available person or thing.

The quality or condition of being available.

The definition of availability is whether someone or something can be accessed or used.

One of the critical missions of the system administrator; that is, to ensure that the computer system not only is available to users 24 hours per day, every day, but also is secure. A system that is shut down may be secure because crackers cannot enter it and do their damage, but the cost to the enterprise can be extreme in terms of lost productivity and sales. For this reason, system administrators act expeditiously in the event of a Denial of Service (DoS) attack. Some safety features are built into secure systems that actually force a shutdown, including fail-close/fail-open, whereby a system shuts down when security features are compromised, such as when a firewall crashes. Another example is account lockouts, which occur when a computer system encounters an onslaught of “bad” passwords, thus locking out the accounts in question.

(countable) That which is available.