Availability (Uptime)

Refers to the amount of time within a 24 hour period a system is active or available for servicing requests.

It is a measure of the time a computer system has been "up" and running. It came into use to describe the opposite of downtime, times when a system was not operational. The uptime and reliability of computer and communications facilities is sometimes measured in nines (similar to the unit of metallic purity).

For example, if a hosting company says it is available 99.999% of the time (or five nines), they are claiming that your web site will up all the time except for about 8 seconds each day. Over the course of a year, in this example, the hosting company is claiming that your site will only be unavailable (unable to view to it) for approximately 48 minutes.