Noun

1. Severe overheating of the core of a nuclear reactor resulting in the core melting and 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. http://www.time.com/time/daily/chernobyl/chernobyl.index.html

2. A situation being likened to a nuclear meltdown.

   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. http://news.independent.co.uk/uk/politics/story.jsp?story=587262

3. A tantrum.