Delusion

A delusion is defined by the Diagnostic and Statistical Manual of Mental Disorders as a belief with no basis in reality, or shared with a person's culture, that persists despite evidence to the contrary. Delusions can be classified as bizarre, if the belief is not plausible or does not occur in ordinary life, and nonbizarre if the situation could possibly happen in reality. A delusion can be both a disorder and a symptom of a disorder such as schizophrenia, Alzheimer's disease, and Parkinson's disease.