MTBF - Mean time between failure
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. 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).
Source: http://en.wikipedia.org/wiki/MTBF
Tutorial: http://www.evaluationengineering.com/archive/articles/0500analyz.htm