Megalomania (from the Greek word μεγαλομανία; megalo-, meaning large, and mania) is a historical term for behavior characterized by an obsession or preoccupation with wealth, power, genius, or omnipotence – often generally termed as delusions of grandeur or grandiose delusions.
Megalomania denotes an obsession with having and/or obtaining, grandiosity and extravagance (especially in the form of great fame and popularity, material wealth, social influence or political power, or more than one or even all of the aforesaid). It may be a symptom of manic or paranoid disorders. However it is not considered a distinct mental disorder of itself according to the Diagnostic and Statistical Manual of Mental Disorders.
Delusions of grandeur, commonly seen in psychosis, may be seen as distinct from megalomania: a megalomaniac’s overwhelming and excessive preoccupation with his or her own importance, though it may be considered pathological, is not necessarily delusional. A delusion of grandeur, if it is a true delusion, must meet the psychiatric criteria for delusion.
Whereas it is possible, in the case of megalomania, for an actually important man/woman to be preoccupied with his/her own actual importance, a person suffering from delusions of grandeur would stubbornly entertain patently false, generally fantastic and often highly complex ideas of his/her own importance, often with a supernatural or science-fictional bent. A person suffering from delusions of grandeur may actually be an important figure, as in the case of the mathematician John Nash, who once rejected a prestigious academic chair on the grounds that he was due to be enthroned as the Emperor of Antarctica.