Delusions are beliefs or ideas that are thoroughly believed to be true, despite being demonstrably false. Often, delusions are symptoms of mental disorders, such as schizophrenia and bipolar disorder. A person with a delusion cannot be convinced that his or her false belief is not true.
Delusions are symptoms of several psychotic disorders: schizophrenia, schizoaffective disorder, delusional
disorder, disorder, shared psychotic disorder, brief psychotic disorder, substance-induced psychotic disorder, bipolar disorder, and depression with psychosis. Delusions are also present in some people who have Alzheimer’s disease
Delusions may also represent a decline in a person with Parkinson’s condition (though they are rare).