Delusion refers to the incorrect belief which is out of keeping with the person's cultural context, intelligence and social background and which is held with unshakeable conviction.
Other /More definition:
Delusion refers to a psychotic symptom involving a false or fixed belief based on incorrect deductions or misrepresentations of reality; these beliefs are not normative within the person’s cultural or religious group; the false beliefs persist despite incontrovertible, obvious proof of their falsity.
Delusion likewise refers to a false belief held against all contrary evidence; of reference false belief that external events, such as people's actions or natural disasters, relate somehow to oneself.
Lexicon of Psychology - Terms, Treatments, Biographies,