the melting of a significant portion of a nuclear-reactor core due to inadequate cooling of the fuel elements, a condition that could lead to the escape of radiation.
a quickly developing breakdown or collapse: a bond-market meltdown; the meltdown of a marriage.
Informal. a sudden loss of control over one’s feelings or behavior: My toddler had a meltdown when I tried to leave the house.

1960–65; noun use of verb phrase melt down Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
meltdown (ˈmɛltˌdaʊn)
1.  (in a nuclear reactor) the melting of the fuel rods as a result of a defect in the cooling system, with the possible escape of radiation into the environment
2.  informal a sudden disastrous failure with potential for widespread harm, as a stock-exchange crash
3.  informal the process or state of irreversible breakdown or decline: the community is slowly going into meltdown

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1965 in reference to a nuclear reactor, from melt + down (adv.). Metaphoric extension since early 1980s.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
meltdown   (mělt'doun')  Pronunciation Key 
Severe overheating of a nuclear reactor core, resulting in melting of the core and escape of radiation.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary

meltdown definition

The most serious accident that can occur at a nuclear reactor. In a meltdown, the radioactive material in the reactor becomes very hot, melting some or all of the fuel in the reactor. A meltdown may or may not be followed by the release of radioactive material to the environment. A partial meltdown, with very little external radiation, occurred at Three Mile Island in 1979; a complete meltdown happened at Chernobyl in 1986.

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Computing Dictionary

meltdown definition

network meltdown

The Free On-line Dictionary of Computing, © Denis Howe 2010
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Example sentences
It's not a movie for the post-meltdown age but one for the post-9/11 age of devil-may-care vengeance.
Virtually anything that caused stress disoriented them, and they quickly descended into pre-meltdown conditions.
Many of these kids have no formal education, because their countries were in a
  complete meltdown when they were growing up.
More recently the overextended housing market collapsed, helping to trigger a
  credit meltdown.
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