Site uptime - this is the time that your website works. Often you can find such a combination of the words as "uptime is 99.9%."
Uptime is a great indicator of how well a web hosting support their systems in working condition. If the hosting provider has a high percentage of uptime, it means that their servers and sites are working too. Therefore, the web-page support is extremely important.
Site uptime is always defined as a percentage of a certain period of time. If the hosting uptime claims that they have 99%, this would mean that 1% is the time when hosting will not work. In a week it will be 1 hour 40 minutes and 48 seconds.
Another way to think about this is to calculate the uptime monetary loss. At low percentage of uptime, your site more often won’t be available, respectively the earnings will be less.