What is an SLA? (service level agreement)
An SLA (service level agreement) is the measurable part of a service contract: how fast issues get a response, how quickly different severities get resolved, what uptime a service promises, and what the remedy is when the promise is missed. "99.9% uptime" allows about 43 minutes of downtime per month; every extra nine multiplies the cost of achieving it.
The numbers that matter
Response time (a human acknowledges), resolution time (it is fixed), and uptime. Response targets are usually tiered by severity: an outage in minutes or an hour, a nuisance in a business day. Uptime percentages translate to concrete minutes: 99% is over 7 hours a month, 99.9% is 43 minutes, 99.99% is 4 minutes. Promising more than the underlying vendors promise you is a common way to lose money.
Related terms: PSA, patch management.