SLA
Also known as: service level agreement
An SLA is a contractual commitment about the service a supplier will provide: how much of the time it will be available, how quickly they will respond to a fault of a given severity, and what the customer is entitled to when the commitment is missed.
The number people quote is availability, and its meaning is worth translating. 99.9% permits roughly forty-three minutes of downtime a month; 99.99% permits about four. Each additional nine costs considerably more to deliver, because it forces redundancy into every layer.
The clauses that matter more in practice are response time and scope. An availability figure that excludes maintenance windows, is measured by the supplier, and compensates with service credits nobody claims is a marketing figure. Support response times are the part a customer actually experiences.