The uptime is the percentage of time during which the host is accessible via the Internet. Most web hosts advertise that they have an uptime of 99% or 99.9%, but this often corresponds to the uptime of the server (ie the server is actually under-voltage), without taking into account the actual availability of the latter on the Internet. The downtime or server maintenance time, planned or otherwise, necessary in any environment, can potentially reach much higher percentages than those guaranteed by the host.

Thus, uptime and accessibility are often included in contracts or SLAs (Service Level Agreements) providers of web hosting. They sometimes even offer a refund or a discount on tarfe if the percentage of uptime promised have not been met.

For information, here is the total duration of unavailability of your website potential the rate of guaranteed uptime from your host: