Uptime

Uptime is how often your site is up and reachable, usually expressed as a percentage. "99.9% uptime" means it's down only a tiny sliver of the time. Its opposite, downtime, is every minute visitors hit an error or nothing loads at all.

People often describe uptime in "nines" — shorthand for how many nines appear in that percentage. "Three nines" is 99.9%, "four nines" is 99.99%, and "five nines" is 99.999%. Each extra nine is a bigger jump than it looks, cutting the allowed downtime by roughly ten times: three nines still permits nearly nine hours of downtime a year, four nines trims that to under an hour, and five nines to about five minutes.

Hosting providers often promise a specific number of nines in their agreements, and approaches like high availability exist to push that number closer to perfect. Each added nine costs more to deliver, so the right target is the one that matches how much downtime your site can actually afford.