Množina: fifth columns
A subversive group that supports the enemy and engages in espionage or sabotage; an enemy in one's midst; SYN. Trojan horse.
Group within a country secretly aiding an enemy attacking from without. The term originated 1936 during the Spanish Civil War, when General Mola boasted that Franco supporters were attacking Madrid with four columns and that they had a “fifth column” inside the city.