1 : an accident in which the core of a nuclear reactor melts and releases radiation
a nuclear meltdown
fears that an accident could cause meltdown
2 a : a very fast collapse or failure
a stock market meltdown
a company experiencing financial meltdown
b [count] chiefly US, somewhat informal : a very fast loss of emotional self-control
After a long day at the beach, our toddler had a major meltdown in the car on the way home.