RTO is sometimes incorrectly defined as meaning Return to Operation and the translation attributed to ITIL. In fact ITIL glossaries define RTO as Recovery Time Objective.
Recovery Time Objective is a term commonly used in Business Continuity Management and ITIL Service Continuity Management to define the elapsed time in which the aim is to get the system or business process up and running again after a disabling incident has occurred.
It can be defined in ITIL terms as
"The maximum time allowed for recovery of an IT Service following an interruption. The Service Level to be provided may be less than normal Service Level Targets. Recovery Time Objectives for each IT Service should be negotiated, agreed and documented",
although in BCM terms it should also be noted that it is an 'objective' and not an absolute.