The Collaborative International Dictionary
fifth column \fifth` col"umn\, n. [from a statement during the Spanish Civil War (1936) that the Falange had four columns of soldiers marching on the city, and a fifth column ``already there'' (i.e. sympathizers inside the Republican lines).]
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.
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.
Douglas Harper's Etymology Dictionary
1936, from Gen. Emilio Mola's comment at the siege of Madrid during the Spanish Civil War that he would take the city with his four columns of troops outside it and his "fifth column" (quinta columna) in the city.
Wiktionary
n. (context dated English) A group of people which clandestinely undermines a larger group, such as a nation, to which it is expected to be loyal.
WordNet
n. a subversive group that supports the enemy and engages in espionage or sabotage; an enemy in your midst [syn: Trojan horse]
Wikipedia
A fifth column is any group of people who undermine a larger group—such as a nation or a besieged city—from within, usually in favor of an enemy group or nation. The activities of a fifth column can be overt or clandestine. Forces gathered in secret can mobilize openly to assist an external attack. This term is also extended to organized actions by military personnel. Clandestine fifth column activities can involve acts of sabotage, disinformation, or espionage executed within defense lines by secret sympathizers with an external force.
Fifth Column, a Canadian all-women experimental post-punk band from Toronto, came about during the early 1980s.
Fifth column refers to a group of people who undermine a larger group from within. It may also refer to:
The Fifth Column is a subgroup of alien Visitors in the 1980s science fiction franchise V. They are depicted as Visitors who oppose the current leadership and plot to rebel against them.
Usage examples of "fifth column".
The Vitons now were aided by a horde of surgically-created quislings, a hapless, helpless, hopeless but dangerous fifth column.
But this would not be a fifth column of traitors, bent on paralyzing a free country.
Xentos was reporting what he had gotten from the Nostori Fifth Column.
All I know is that some weeks ago he broke up and exposed a well-organised Japanese Fifth Column in Eastern Borneo.