meltdown Syllabification: (melt·down)Pronunciation: /ˈmeltˌdoun/Translate meltdown | into French | into German | into Italian Definition of meltdown noun 1a disastrous event, especially a rapid fall in share prices:the 1987 stock market meltdown informal an outburst of severe emotional distress; a nervous breakdown:they wondered what could have triggered her meltdown 2an accident in a nuclear reactor in which the fuel overheats and melts the reactor core or shielding. meltdown in other Oxford dictionaries Definition of meltdown in the British & World English dictionary