Short answer
Uptime is the percentage of time a service is available; a service level agreement is the contract stating the target and what happens when it is missed. The numbers are less generous than they look: 99.9% uptime permits roughly 43 minutes of downtime a month, and 99% permits over seven hours.
Also called: SLA, Uptime guarantee, Availability
Read the exclusions before the percentage. Most agreements measure availability from the provider’s own monitoring, exclude scheduled maintenance windows, and disclaim outages originating with third parties such as a payment gateway or a DNS provider. A figure calculated that way can look healthy through an afternoon when customers could not check out.
The remedy is usually a service credit against the next invoice rather than compensation for lost trade, which is why the response commitments matter more than the nines for most small businesses. Who answers outside office hours, how quickly, and through which channel are the terms that decide how a Saturday outage actually goes. Independent monitoring you control is worth having, so the conversation starts from your own record rather than the provider’s.
Common questions
What uptime should we ask for?
For most business sites, 99.9% is a reasonable target and anything higher adds cost quickly through redundancy. Spend the negotiation on response times and escalation instead: a documented out-of-hours contact and a committed response window prevent more damage than an extra decimal place in the availability figure.
Related terms
Web Hosting
Web hosting is the paid service that keeps a website’s files on a server connected to the internet so visitors can load them.
SSL/TLS Certificate
An SSL/TLS certificate is a file installed on a server that encrypts traffic between a browser and a website, turning http:// into https:// and removing the browser’s "Not secure" warning.
Legacy System Migration
Legacy system migration is moving a business off software it has outgrown, such as an unsupported CMS version or a custom application nobody remembers building, onto a current platform with its data intact.
Reading definitions because you are scoping a project? Skip ahead and just ask.