Mean Time Between Failures (MTBF)
MTBF is a basic measure of reliability for repairable items.
It can be described as the number of hours that pass before
a component, assembly, or system fails. It is a commonly-used
variable in reliability and maintainability analyses.
MTBF can be calculated as the inverse of the failure rate
for constant failure rate systems. For example: If a component
has a failure rate of 2 failures per million hours, the
MTBF would be the inverse of that failure rate.
MTBF = (1,000,000 hours) / (2 failures) = 500,000 hours
more information: http://www.i-mtbf.com/