Megalomania implies an unhealthy obsession with power, showing someone's overconfidence in their authority. The dictator's speech reeked of megalomania, with claims of ruling the world.
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Negative Connotation
Megalomania usually suggests a person's ambition has become harmful or domineering, lacking empathy. His megalomania was evident when he dismissed all advice that contradicted his opinions.
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Loss of Reality
Note that megalomania involves losing touch with reality, not just arrogance or confidence. She was accused of megalomania after declaring herself infallible.