A delusion is a strongly-held or fixed false belief that conflicts with reality. The Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5) defines delusions as fixed beliefs that are not amenable to change in light of conflicting evidence.

A delusion is an unshakable belief in something that’s untrue. The belief isn’t a part of the person’s culture or subculture, and almost everyone else knows this belief to be false. People with delusional disorder often experience non-bizarre delusions.