Basic definitions -- 4
Maintainability, M(t) -- the probability that a failed system will be restored to operation within time t. M(t) = 1 - e-?t.
Mean time to repair, MTTR -- the expected value of system repair time. MTTR = 1/?.
Mean time between failures, MTBF -- the expected value of the time between failures for a system with repair. MTBF = MTTF + MTTR for systems with constant rates.