Step 1: To classify the symptom, identify whether the disturbance lies in belief or in perception. This case is about a conviction the patient will not give up.
Step 2: She is certain of her husband's infidelity, dismisses every piece of contrary evidence, and her family does not share the idea. A rigidly held, false, culturally unshared belief of this kind is by definition a delusion, and a jealousy theme makes it a delusion of infidelity.
Step 3: It is not an illusion (a real stimulus misread) nor a hallucination (a perception with no stimulus at all), and perversion concerns abnormal behaviour rather than belief.
Step 4: The disorder of thought content, not of perception, fixes the diagnosis.
\[\boxed{\text{Delusion}}\]