The Free On-line Dictionary of Computing (30 December 2018):

Mean Time Between Failures
Mean Time Between Faults
MTBF
TBF

    (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)