These terms define reliability expectations:
SLI (Service Level Indicator): The metric you measure. Example: Request latency, error rate, availability.
SLO (Service Level Objective): Your internal target. Example: % of requests under ms.
SLA (Service Level Agreement): External commitment with consequences. Example: % uptime or customer gets credits.
Relationship: SLOs should be stricter than SLAs. If SLA is %, set SLO at % to have buffer.
Error budgets: If SLO is %, you have % error budget. Spend it on deployments, experiments, or save for incidents.