MTTR
MTTR (Mean Time To Restore) is the amount of time required to get back up and running. This is sometimes called Mean Time To Repair.
MTBF
MTBF (Mean Time Between Failures) is a prediction of how long the system will be operational before a failure occurs.
RPO
An RPO (Recovery Point Objective) is a qualifier that determines when the system is recovered. A recovered system may not be completely repaired, but it will be running well enough to maintain a certain level of operation.
MTTF
An MTTF (Mean Time To Failure) is the expected lifetime of a nonrepairable product or system.