Fifth column stands for any clandestine group of people which works inside a nation to undermine its strength, while at the same time the nation suffers a direct attack from a foreign power.

The term was coined by Gen. Mola during the Spanish Civil War (19361939). As a Nationalist general, he sent four of his army columns to capture Madrid, which was being defended at the time by the Republican forces. The general referred to his hidden supporters inside the capital as his fifth column.