[pur-muh-frawst, -frost]
(in arctic or subarctic regions) perennially frozen subsoil.
Also called pergelisol.

1943; perma(nent) + frost Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
permafrost (ˈpɜːməˌfrɒst)
ground that is permanently frozen, often to great depths, the surface sometimes thawing in the summer
[C20: from perma(nent) + frost]

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

1943, coined in Eng. by S.W. Muller from perm(anent) frost.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
permafrost   (pûr'mə-frôst')  Pronunciation Key 
A layer of soil or bedrock that has been continuously frozen for at least two years and as long as tens of thousands of years. Permafrost can reach depths of up to 1,524 m (4,999 ft). It is found throughout most of the polar regions and underlies about one fifth of the Earth's land surface.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
It is the largest river in the world that is completely underlain by permafrost.
They show a surface broken into polygonal slabs by repeated freezing and
  thawing of the sort that happens above permafrost.
Vast reaches of the planet have been locked for millennia in stunning
  permafrost formations.
Thawing permafrost may soak up greenhouse gas before it begins to spew it out.
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