Service Level Objective
also called SLO
An internal target for a service level indicator, set below the level at which users notice, and used to decide whether to ship or to stabilise.
Three related terms, routinely conflated. An SLI is the measurement — the proportion of requests served successfully in under 300 ms. An SLO is the target for it — 99.9% over 30 days. An SLA is a contract with a customer that includes a penalty, and it should always be looser than the SLO, so that missing the target internally is a signal rather than an invoice.
The discipline an SLO introduces is that reliability becomes a number with a budget rather than an aspiration. 99.9% over 30 days permits 43 minutes of failure. That is the error budget, and it is what makes "should we ship this risky change" answerable with evidence.
Setting the target too high is the common error and it is expensive: each additional nine typically costs an order of magnitude more, and users rarely perceive the difference above the point where their own network is the dominant term.