Severe overheating of the core of a nuclear reactor resulting in the core melting and potentially in radiation escaping.
Four years have passed since the meltdown at the Chernobyl nuclear plant, but the grim legacy of the Soviet catastrophe is still unfolding. https://web.archive.org/web/20041130094534/http://www.time.com/time/daily/chernobyl/chernobyl.index.html
A situation being likened to a nuclear meltdown; a crisis.
Computer engineers were at a loss last night to explain why the Government had been hit by arguably the worst electronic meltdown in the history of Whitehall. https://web.archive.org/web/20041209011615/http://news.independent.co.uk/uk/politics/story.jsp?story=587262
(informal) A tantrum or emotional outburst.
(psychology) An autistic response to stress or sensory overload, in which the person is overwhelmed by intense, seemingly disproportionate emotions, behaving erratically and becoming unable to mask.
Now if I am really anxious, I can take a moment to think about what is making me anxious and what to do about it; before I would just spiral into a meltdown really quickly, with no understanding of why.