A service-level indicator (SLI) is a metric that is used to measure the performance of a service against a defined service-level objective (SLO). SLIs are typically used in conjunction with SLAs and SLOs to provide a clear and measurable way to track the quality of the service being provided. Examples of common SLIs include availability, response time, and error rate. By monitoring these metrics, service providers and customers can ensure that the service is meeting the agreed-upon SLO.