Mean Time Between Failures(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}. (1998-05-01)