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English definition of “fifth column”

fifth column

noun [C] uk   us  
a group of people who support the enemies of the country they live in and secretly help themDisloyalty, betrayal and treason
fifth-columnist
noun [C] uk   us  
a member of such a groupDisloyalty, betrayal and treason
(Definition of fifth column from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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