Service level agreement (SLA)
The written commitment to how fast a provider will respond, and what happens if they do not.
The number that matters is response time, not resolution time, and reputable agreements only commit to the first. Resolution depends on vendors, hardware and access that no provider controls, so a guaranteed fix time is either heavily caveated or not credible.
Read the hours the SLA applies to before the numbers inside it. A one-hour response is meaningless if the clock only runs Monday to Friday, nine to six, and your problem is on Saturday. That is not a criticism of business-hours cover, which is fine and cheaper, as long as it is stated plainly.
The other thing to check is the remedy. Many SLAs commit to a target with no consequence attached, which makes it a statement of intent rather than an agreement.
What it costs
Business-hours response is the standard inclusion. Guaranteed one-hour senior response outside that is typically sold per incident, around $150.
Related questions
What is a good response time for IT support?
Same business day is normal for small-office plans and usually much faster in practice. Sub-hour guarantees generally mean either a premium retainer or a per-incident escalation fee.
Does an SLA guarantee my problem gets fixed in time?
Almost never, and be sceptical of one that claims to. Credible agreements commit to a first response, because resolution depends on vendors and hardware nobody controls.