delusion

The delusion - A fixed, irrational idea not shared by others and not responding to reasoned argument. The central idea in a paranoid delusion involves persecution or jealousy; for instance, a person may falsely believe that he or she is being poisoned or that a partner is persistently unfaithful (see paranoia).

Persistent delusions are an indication of serious mental illness, particularly schizophrenia and manic–depressive illness. (See also hallucination; illusion.)