meltdown

UK: /ˈmɛltdaʊn/ US: /ˈmɛltˌdaʊn/ ,(melt′doun′)

WordReference Random House Learner's Dictionary of American English © 2025

melt•down /ˈmɛltˌdaʊn/   n. [countable]

1. Physics, Energy the melting of the core of a nuclear reactor, due to inadequate cooling of the fuel elements.
2. any quickly developing breakdown, mishap, or accident.

WordReference Random House Unabridged Dictionary of American English © 2025

melt•down  (melt′doun′), n. 

1. Physics, Energy the melting of a significant portion of a nuclear-reactor core due to inadequate cooling of the fuel elements, a condition that could lead to the escape of radiation.

• noun, nominal use of verb, verbal phrase melt down 1960–65

Collins Concise English Dictionary © HarperCollins Publishers::

meltdown / ˈmɛltˌdaʊn/ noun

1. (in a nuclear reactor) the melting of the fuel rods as a result of a defect in the cooling system, with the possible escape of radiation into the environment
2. a sudden disastrous failure with potential for widespread harm, as a stock-exchange crash
3. the process or state of irreversible breakdown or decline: the community is slowly going into meltdown