The observed way or mechanism of failure. For a failure mode to be valid it must answer the question:
“What is the primary manner in which the associated failure descriptor becomes evident?”
Depending on the application, the definition of failure mode may slightly vary. A failure mode can refer to a physical event/mechanism that gave rise to a failure, like moisture corrosion, fatigue, or wear. It can also come from a functional point of view; a valve can have several failure modes such as fails to operate on demand, valve leakage, etc. In a formal reliability-centered maintenance program, deeper failure modes are considered as failure causes.