Availability

Availability is a term used in information security to show that a system is running and reachable by those that are supposed to be able to use the system.
Availability is sometimes expressed as a percentage of uptime, such as 99%. High availability is usually expressed as five nines - 99.999%. In a 24-hour, 7-day-a-week operation this would be just a little over 5 minutes per year.