Availability

Availability or service availability is the amount of time that a service is up and functional. It is often expressed as a percentage uptime (such as 99.9%) over a period of time.

availability = available time / total time

A service that has a 99.9% ("three nines") availability over a year is guarenteed to only have 8.77 hours minutes of downtime.

Depending on the type of service, the availability guarentees may vary. 99% availability may be okay for batch jobs. But higher availability guarentees may be expected for services providing online banking or POS transactions.