A written commitment from a provider to a defined performance level, such as uptime and response time for incidents.
SLA stands for service level agreement. It is the part of a contract that states what the provider guarantees: the share of time the service runs without interruption, how fast issues are handled by severity, and what the customer receives, such as credits, if the commitment is missed.
When your business depends on an external service like hosting, a payment gateway or messaging, the SLA tells you what you can realistically expect. Large companies may also ask you for an SLA if you sell them software. See enterprise solutions and the trust center.
Practical tip: read the details, not just the headline: how is uptime measured, what is excluded, such as scheduled maintenance, and how do you claim credits? Never promise your customers a higher service level than your own providers guarantee you.