1. severe overheating of a nuclear reactor core, resulting in melting of the core and escape of radiation.
2. informal A disastrous or rapidly developing situation likened to the melting of a nuclear reactor core: "after several corporate meltdowns, only two reporters remain in [the] bureau” ( david Fitzpatrick).
3. informal an emotional breakdown

1. severe overheating of the core of a nuclear reactor resulting in the core melting and radiation escaping.
2. A situation being likened to a nuclear meltdown; a crisis.
3. A tantrum.

1. severe overheating of the core of a nuclear reactor resulting in the core melting and radiation escaping
2. a disaster comparable to a nuclear meltdown