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Definition of permafrost noun from the Oxford Advanced Learner's Dictionary

 

permafrost

 noun
noun
BrE BrE//ˈpɜːməfrɒst//
 
; NAmE NAmE//ˈpɜːrməfrɔːst//
 
, NAmE//ˈpɜːrməfrɑːst//
 
[uncountable] (specialist)
 
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a layer of soil that is permanently frozen, in very cold regions of the world Word Origin 1940s: from permanent + frost.
See the Oxford Advanced American Dictionary entry: permafrost