fifth column

WordReference Random House Unabridged Dictionary of American English © 2025

fifth′ col′umn, 

1. Government a group of people who act traitorously and subversively out of a secret sympathy with an enemy of their country.
2. Government (originally) Franco sympathizers in Madrid during the Spanish Civil War: so called in allusion to a statement in 1936 that the insurgents had four columns marching on Madrid and a fifth column of sympathizers in the city ready to rise and betray it.
fifth′ col′umnist. 

Collins Concise English Dictionary © HarperCollins Publishers::

fifth column noun

1. (originally) a group of Falangist sympathizers in Madrid during the Spanish Civil War who were prepared to join the four columns of insurgents marching on the city
2. any group of hostile or subversive infiltrators; an enemy in one's midst

fifth columnist noun