As refering to a website or ISP.
Service Level Agreement? 
People usually talk in terms of availability, which is defined as the percentage of time a system is available for production use. This is distinguished from reliability, which measures failure rates, and servicibility, which measures how long it takes to restore a down system to use.
People also talk about mean time to failure, and mean time between failures. There is also a measurement called FIT (failures in time) which is the number of failures per billion hours of operation.
Availability is usually the best measure. If two system fails once a month, they both have the same reliability. If one takes a day to bring back up, and the second takes a minute, the second system has far better availability.
Another acronym is LOS (Level of Service).
Nah… the SLA is the overall level of service agreed to in a contract. It includes many other things related to service, such as help desk response times, help desk hours, changes to server configurations, etc. Uptime is an important part of the SLA, but isn’t the only one.
As to the OP, I always just called it “uptime” and no one’s ever looked at me funny.
Now what company can profess “zero downtime”? Normally availability is measured using the “nines”… as in 99.9999% uptime for example. Sorry if this seem pendantic.
For airliners, it’s “availability”.
I don’t think there is a term for this because no company would be foolish enough to make such a guarantee. SLA’s don’t guarantee up time; they outline what restitution will be offered in the event certain service levels aren’t met. Some of the service levels measured are (1) mean time to repair (MTTR), (2) network availabillity and (3) meeting installation dates.
Perhaps not guaranteed, but definitely promised. Customers whose systems are down more than the guarantee are often very unhappy, and restitution doesn’t change that.
I’m not sure I see an important difference between a promise and a guarantee when you are talking to a customer. To promise 100% uptime is just as foolish as to guarantee it. I interface with 8 blue chip customers and none of them have expressed the notion that 100% uptime is a service requirement. And with only one exception, all of them have redundant lines in anticipation of service interruptions.
Contractors cut through our cables, manholes flood and cars knock down telephone poles. That’s just the way it is.
I was speaking as the telco, and the ISP is our customer.
I see the OP was talking about ISP’s. An ISP might be able to get away with a 100% uptime assurance if they had a lot of redundancy. If a telco line is cut, they could re-route traffic through an alternate path and continue to provide service to their customers. But if I was the ISP, I’d still worry that I had a possible single point of failure somewhere.
Five nines, six nines, etc.