A deviation from the specified or expected behavior that is visible to end-users is called:

A deviation from the specified or expected behavior that is visible to end-users is defined as a "failure." According to the ISTQB CTFL Syllabus, version 4.0:

Anerroris a human action that produces an incorrect result.

Afault(also known as a defect or bug) is a flaw in a component or system that can cause the system to fail to perform its required function.

Afailureoccurs when a fault is executed, leading to a deviation from the expected behavior, and this deviation becomes apparent to the end-users.

Thus, a visible deviation from the specified or expected behavior is termed a failure in the context of software testing​​.