Uptime
The proportion of time a system is available, usually expressed as a percentage over a period.
The nines are less generous than they sound. 99.9% permits about 8.8 hours of downtime a year, 99.99% about 53 minutes. Rounding a service level up in conversation hides a meaningful difference.
What the figure covers matters as much as the number. Availability measured excluding planned maintenance is a different promise from availability measured against real usable time.
Where this comes up
Uptime sits inside our networks and infrastructure work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.