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.
- A group of people which clandestinely undermines a larger group, such as a nation, to which it is expected to be loyal.
- 2020 January 2, Conrad Landin, “Strife and strikes in post-war Britain”, in Rail, page 52:
- Lloyd George's belief in a revolutionary fifth column [the term hadn't been coined then] driving the strike was replicated on Fleet Street. On October 1  The Times thundered: "The attempt of the strike leaders to subvert Constitutional government in this country and to starve the nation into acquiescence in their revolutionary designs will be defeated, as other onslaughts have been defeated, by the firmness of the people in defence of its liberties."
- Used other than figuratively or idiomatically: see fifth, column.