An SLA (service level agreement) is an agreement between provider and client about measurable metrics like uptime, responsiveness and responsibilities. You can see a sample service level agreement here.
1.1 The SLA MUST be between a vendor and a paying customer.
1.2 The SLA MUST be reviewed by both legal and IT operations.
1.3 The SLA MUST include a service description that specify which system parts are included in your agreement and which are out of scope. (third party systems are most often out of scope)
2 Service Level Indicators
An SLI (service level indicator) is a defined quantitative measure of the level of service that is provided.
2.1 All metrics MUST be something you can measure and create automated reports for. It should not require manual steps to create a report.
2.2 All metrics MUST be specified how it should be measured
2.2.1 Use aggregates for time intervals or regions