How much uptime a service has, including scheduled downtime.
Not to be confused with Reliability which doesn't include scheduled downtime. For example "Sunlight" is very Reliable (it comes up every day!) but is only about 50% Available (it's dark lots too, but people expect that).
Note, rebooting your machine once a week to get rid of any "issues" that might have cropped up, means your system is very Reliable (assuming no issues actually occur) but not that Available.