Observability
A written promise in a contract about how well a service will work, with a price to pay, such as money back, if it is broken.
Formal
The part of a contract between a supplier and a customer that states measured levels of service, such as the share of time a service is in use or how fast a fault must be answered, and what the supplier owes when they are not met.
In plain English
Like a pizza place that promises delivery in thirty minutes or the pizza is free; the promise only means something because breaking it costs them.
In practice
A Danish shipping company's contract with its hosting supplier promises 99.9% availability each month; after a four-hour outage, the company gets a tenth of the monthly bill back as agreed.
Why it matters
When you depend on a supplier, the SLA is what you can actually hold them to, so it must match what your own business and your own promises to others need.