Megalomania is the belief that you are more powerful and important than you really are. Megalomania is sometimes a mental illness. when someone wants to have a lot of power for themselves and enjoys having control over other people's lives, sometimes as part of a mental illness (megalo- (from megas; MEGA-) + mania)
disapproval If you describe someone as a megalomaniac, you are criticizing them because they enjoy being powerful, or because they believe that they are more powerful or important than they really are. someone who wants to have a lot of power for themselves and enjoys having control over other people's lives