a group of people who aid
the enemy from within their own country
Webster’s New World College Dictionary, 4th Edition. Copyright © 2010 by
Houghton Mifflin Harcourt. All rights reserved.
Word origin of 'fifth column'transl. of Sp quinta columna: first used (1936) by the Sp Nationalist General Mola, who, besieging Madrid with
four columns from the outside, boasted of having a “fifth column” of civilian supporters