Qu'est-ce que « Mean time between failures, mean time to repair » ?

The mean time between failures (MTBF) of a component is the average time it runs between failures; its mean time to repair (MTTR) is the average time from a failure to its restoration. Its steady-state availability is $\mathrm{MTBF}/(\mathrm{MTBF}+\mathrm{MTTR})$.