Recovery Time Objective (RTO)
The maximum acceptable length of time your systems can be down after a failure before the impact becomes unacceptable to the business.
Definition
Recovery Time Objective (RTO) is the maximum amount of time your systems can remain unavailable after a disruption before the outage causes unacceptable damage to the business. It's one of the two numbers, alongside Recovery Point Objective, that define the shape of a disaster recovery strategy. A lower RTO demands more automated, more expensive infrastructure; a higher RTO allows for slower, cheaper recovery paths. Neither number is right or wrong on its own; it depends entirely on what a given workload can tolerate.
In Practice
A finance workload with a 4-hour RTO needs automated failover in place. A weekly reporting workload with a 48-hour RTO can rely on a standard restore-from-backup process instead: the acceptable downtime, not the technology, is what should set the target.