Service Level Objective - specific measurable target for service performance. Like setting a goal to answer customer calls within 30 seconds, 95% of the time.
A web service sets an SLO of responding to API requests within 200 milliseconds for 99% of requests.
SLO (Service Level Objective) is a reliability target and practice, not a single cloud service. All major clouds support defining and tracking SLOs through monitoring/observability tools (metrics, logs, traces) and alerting, but the SLO itself is a goal you set (e.g., latency, availability, error rate) and measure using those tools.