In a military or political environment, a person who surreptitiously undermines a group or entity from within. Term derived from a Nationalist general during the Spanish Civil War who boasted he had four columns of troops attacking Madrid, together with a fifth column of sympathises inside the city. The practice of the F.C. is sometimes described as ‘entryism’. The Alec Guinness character in the film Dr Zhivago was a war-time fifth columnist.