SRE - Site Reliability Engineer (a term Google came up with that's been adopted elsewhere) Google defined it approximately as what happens when you apply software engineering practices to what was traditionally an operations function.

SLO - Service Level Objective - the service level you strive for. If it's higher you have room for experimentation, etc.

SLI - Service Level Indicator - the actual metric(s) you use to measure a service level (latency, error rate, throughput, etc.)

SLA - correct. That’s the contract between the operator and the users which describes the penalties for not meeting agreed-upon SLO

SLO - service level objective, the stated availability (or latency or durability etc) of the service. Usually expressed as a value over a period of time (e.g 99.9% availability as measured over a moving 30 average). The SLO is measured by the SLI.

SLI - service level indicator. Simply, the direct measurement of the service (i.e metrics)

SRE - Site Reliability Engineer, usually a member of a team who is responsible for the continued availability of the service and the poor sap who gets paged when it breaches SLO or has an outage or other impactful event.

SRE: Site reliability engineer

SLI: Service level indicator (Metric to measure the health of a service. For example successful requests per interval / total requests per interval.)

SLO: Service level objective (what performance you expect eg. the previously mentioned SLI is >= 99.5%)

SLA: Servicelevel agreement (legal agreement that defines what happens if a SLO is not met)