fifth column

Từ điển Oxford Advanced Learner 8th

ˌfifth ˈcolumn BrE NAmE noun
a group of people working secretly to help the enemy of the country or organization they are in
Derived Word: ↑fifth columnist

Word Origin:
The 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.