Definition of “fifth column” - English Dictionary

“fifth column” in British English

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fifth columnnoun [ C ]

uk /ˌfɪfθ ˈkɒl.əm/ us /ˌfɪfθ ˈkɑː.ləm/

a group of people who support the enemies of the country they live in and secretly help them

fifth-columnist
noun [ C ] uk us

a member of such a group

(Definition of “fifth column” from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)