Source : Free On-Line Dictionary of Computing

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)