When what is visible to end-users is a deviation from the specific or expected behavior,...

When what is visible to end-users is a deviation from the specific or expected behavior, it is called a failure. A failure occurs when the software does not perform a function as expected or produces incorrect results, which can be observed by the users. This is distinct from an error, which is a human action that produces an incorrect result, a defect (or bug), which is a flaw in the software, or a fault, which is another term for a defect​​.