Fault vs Failure

Understanding the distinction between fault and failure is foundational in dependability, reliability, and software quality engineering. In precise dependability terminology, a fault is the cause of an error, while a failure is the externally visible event in which delivered service deviates from correct service. This distinction matters because systems can contain many faults yet exhibit no immediate failures if those faults remain dormant, are never activated, or are masked by fault-tolerance mechanisms.

A practical mental model is:

• A human mistake may introduce a defect or fault in design, code, configuration, or hardware.
• When the fault is activated, it can create an erroneous internal state.
• If that erroneous state propagates to the service interface, users observe a failure.

This causal