MTTF

Failure rate is the frequency with which an engineered system or component fails, expressed for example in failures per hour. It is often denoted by the Greek letter λ (lambda) and is important in reliability theory. In practice, the reciprocal rate MTBF is more commonly expressed and used for high quality components or systems.
See more at Wikipedia.org...

Mean time between failures (MTBF) is the mean (average) time between failures of a system, and is often attributed to the "useful life" of the device i.e. not including 'infant mortality' or 'end of life'. Calculations of MTBF assume that a system is "renewed", i.e. fixed, after each failure, and then returned to service immediately after failure. The average time between failing and being returned to service is termed mean down time (MDT) or mean time to repair (MTTR).
See more at Wikipedia.org...

MEAN TIME TO FAILURE. MTTF