Grandiose delusion or delusions of grandeur is principally a subtype of delusional disorder (GD) that can occur as a wide range of mental illness, including in two thirds of those in manic state of bipolar disorder, half those with schizophrenia and a substantial portion of those with substance abuse disorders.[1][2] GDs are characterized by fantastical beliefs that one is famous, omnipotent, wealthy, or otherwise very powerful. The delusions are generally fantastic and typically have a supernatural, science-fictional, or religious theme. There is a relative lack of research into GD, in comparison to persecutory delusions and auditory hallucinations. About 10% of healthy people experience grandiose thoughts but do not meet full criteria for a diagnosis of GD.[2]
Grandiose delusions are distinct from grandiosity, in that the sufferer does not have insight into his loss of touch with reality.
In colloquial usage, one who overestimates one's own abilities, talents, stature or situation is sometimes said to have 'delusions of grandeur'. This is generally due to excessive pride, rather than any actual delusions.
Also, Smith, Freeman et al. have noted that ref> Smith, Nicola; Simon McCarthy-Jones, Georgina Rowse (2005-07). "Grandiose Delusions: An Experimental Investigation of the Delusion as Defense". Journal of Nervous and Mental Disease 193 (7): 480–487. http://journals.lww.com/jonmd/Abstract/2005/07000/Grandiose_Delusions__An_Experimental_Investigation.8.aspx. Retrieved 2011-10-10.</ref>