Definition of fifth column noun from the Oxford Advanced Learner's Dictionary

 

fifth column

 noun
noun
BrE
 
; NAmE
 
 
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a group of people working secretly to help the enemy of the country or organization they are in Word OriginThe term dates from the Spanish Civil War, when General Mola, leading four columns of troops towards Madrid, declared that he had a fifth column inside the city.
See the Oxford Advanced American Dictionary entry: fifth column