A deviation from the specified or expected behavior that is visible to end-users is referred to as what?

The term that best describes a deviation from the specified or expected behavior that is visible to end-users is a failure. A failure occurs when the software does not perform as intended under specified conditions, resulting in an observable incorrect or unexpected behavior. This observable behavior is what users notice and can lead to dissatisfaction or confusion regarding the software's functionality.

In the context of software testing, the distinction is important because while a fault (or bug) may exist within the code (a defect), it is not considered a failure unless it manifests during operation and is experienced by the end-users. Thus, the failure is the actual manifestation of a problem in a way that is perceivable to users, highlighting the practical impact of issues in software delivery.