A nuclear meltdown is an informal term for a severe nuclear reactor accident that results in core damage from overheating. The term is not officially defined by the International Atomic Energy Agency or by the U.S. Nuclear Regulatory Commission. However, it has been defined to mean the accidental melting of the core of a nuclear reactor, and is in common usage a reference to the core’s either complete or partial collapse. “Core melt accident” and “partial core melt” are the analogous technical terms. For a technical overview, see the behavior of nuclear fuel during a reactor accident.