from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A clandestine subversive organization working within a country to further an invading enemy's military and political aims.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A group of people which clandestinely undermines a larger group, such as a nation, to which it is expected to be loyal.
from the GNU version of the Collaborative International Dictionary of English
- n. a group of persons inside the battle lines of a territory engaged in a conflict, who secretly sympathize with the enemy, and who engage in espionage or sabotage; -- sometimes also referred to as a trojan horse.
- n. Hence, any faction of persons within a group who secretly sympathize with an enemy, especially those who engage in activities harmful to the group; an enemy in one's midst; a group of traitors.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. a subversive group that supports the enemy and engages in espionage or sabotage; an enemy in your midst
First applied in 1936 to rebel sympathizers inside Madrid when four columns of rebel troops were attacking that city.(American Heritage® Dictionary of the English Language, Fourth Edition)
From a 1936 radio address by Spanish Nationalist general Emilio Mola, in which he spoke of four of his army columns moving on Madrid and a quinta columna consisting of his militant supporters within the capital, intent on undermining the Republican government from within. (Wiktionary)