Failure rate
Mean Time Between Failures (MTBF) is closely related to failure rate.  In the special case when the likelihood of failure remains constant with respect to time (for example, in some product like a brick or protected steel beam), and ignoring the time to recover from failure, failure rate is simply the inverse of the Mean Time Between Failures (MTBF).  MTBF is an important specification parameter in all aspects of high importance engineering design–such as naval architecture, aerospace engineering, automotive design, etc.–in short, any task where failure in a key part or of the whole of a system needs be minimized and severely curtailed, particularly where lives might be lost if such factors are not taken into account. These factors account for many safety and maintenance practices in engineering and  industry practices and government regulations, such as how often certain inspections and overhauls are required on an aircraft.