melt ‧ down /ˈmeltdaʊn/ BrE AmE noun [uncountable and countable]
1 . a very dangerous situation in which the material inside a ↑ nuclear reactor melts and burns through its container, allowing ↑ radioactivity to escape
2 . a situation in which prices fall by a very large amount or an industry or economic situation becomes much weaker:
The stock market crash might lead to financial meltdown.