1. An event where the core of a nuclear reactor is heated so much that it melts. A meltdown is considered a catastrophic event that should by all means be avoided as it can cause radioactive materials to be released into the environment.
2.Events that end catastrophically, as if it were as bad as a nuclear meltdown. For example, a person in an unstable and stressful emotional state may state that he or she is experiencing an emotional meltdown.