Megalomania is a hunger for power and greatness that has grown out of proportion — the state of a person who believes they are far more important than they are, wants control over other people, and enjoys having it. The word does two jobs. In everyday use it is a criticism, usually of a leader, a boss, or a rich person whose ambition has stopped being reasonable: 'the project collapsed into pure megalomania'. In medicine and psychology it points to something more serious, delusions of grandeur — a genuine mental illness in which a person truly believes they are a king, a god, or the saviour of the world. Ordinary speakers borrow the medical weight to make the criticism sting, which is why calling someone's plan megalomania is a strong thing to say, not a light tease. It is a formal, written-feeling word: you meet it in journalism, history, and film reviews far more than in conversation. It is uncountable — 'his megalomania', never 'a megalomania'.

誇大狂、權力妄想（醫學上指「誇大妄想」；日常用來罵人權力慾過度膨脹、自我感覺良好到病態）