Glossary
Also: Service level objective · Service level agreement
A measurement (SLI), a self-imposed target for it (SLO), and a contractual promise with consequences (SLA).
The measurement is concrete, for example the share of requests under 300 milliseconds. The objective sets how much of that should be achieved, for example 99.5 percent per month. The agreement turns it into a contractual obligation, usually with credits when missed.
The gap between objective and one hundred percent is the error budget: how much disruption you can afford per month. It is the most useful of the three, because it makes speed and stability tradeable against each other.