Fixed fantasy
A fixed fantasy is a belief or system of beliefs held by a single individual to be genuine, but that cannot be verified in reality. The term is typically applied to individuals suffering from some type of psychiatric dysregulation, most often a personality disorder.
A fixed fantasy differs from a delusion or delusional system in that, superficially, a fixed fantasy tends to appear plausible, and the person expressing the fantasy is not suffering a break from reality, as occurs in a delusional state.
A fixed fantasy also differs from a religion or superstition in that these are culturally bound, whereas a fixed fantasy is specific to an individual.
In and of themselves, fixed fantasies are typically not harmful, but they can sometimes interfere with an individual's ability to develop a coherent and integrated life experience.