Mean Time Between Failures definition specification
(MTBF, or "Mean Time Between Faults") The average
time (usually expressed in hours) that a component
works without failure. It is calculated by dividing the total number of failures into the total number of operating hours observed. The term can also mean the length of time a user may reasonably expect a device or system to work before an incapacitating fault occurs.
See also Mean Time To Recovery