Source : Webster's Revised Unabridged Dictionary (1913)

Megalomania \Meg`a*lo*ma"ni*a\, n. [NL., fr. megalo- + mania.]
   (Pathol.)
   A form of mental alienation in which the patient has
   grandiose delusions.

Source : WordNet®

megalomania
     n : a psychological state characterized by delusions of grandeur