SLI: the measurement
Service Level Indicator = a specific numeric measurement. Examples: fraction of requests with status < 500, p95 latency of POST /checkout, % of pages with first-byte under 200ms. Picking the SLI is the hard part — it must match user pain.
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SLO: the target
Service Level Objective = the SLI's target threshold. 99.9% of requests have status < 500 over 30 days. Two parameters: target value (99.9%) and window (30 days). Tighter than SLA — internal goal, not commercial promise.
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SLA: the commercial promise
Service Level Agreement = a customer-facing contract with refund/credit penalties. If uptime drops below 99.5% in a month, customer gets 10% credit. Should always be looser than your SLO; the gap is your safety margin.