A fifth columnist is a person who is collaborating with an enemy or foreign power with the intention of overthrowing or somehow subverting the current leadership. The term comes from the idiom "the fifth column," based on comments by General Emilio Mola during the Spanish Civil War. He described a group of rebels loyal to his cause who were already present inside the city he sought to conquer. The term has seen use several times since then, and a fifth columnist is now more generically defined as anyone who is fighting for, or actively supporting, the enemy.
Mola was attempting in 1936 to lay siege to Madrid, Spain, during the Spanish Civil War. He had surrounded the city with four columns of his soldiers. During a radio address, he said his four columns of soldiers would be aided by another column of people already inside of the city. The Fitchburg Sentinel newspaper in Massachusetts then used the term "fifth column" in an article about the ultimately unsuccessful siege on 14 October 1936.
The actual fifth columnist group to which Mola was referring to was a faction of residents who lived in Madrid and were opposed to the loyalist government. The rebels formed a secret group and were intent on providing support for the troops that Mola planned to send into the city. Despite the staying power of the idiom, the actual fifth column was not effective in winning control of Madrid.
The term gained popularity, especially during World War II. Both the British and the United States used the term to describe suspicious citizens and immigrants who were of German decent and believed to be sympathizing with Germany. It also was used in Eastern Europe during the same time period to denote groups of Polish and Czechoslovakian citizens who aided in the capture of their own nations by Germany.
The term fifth column is an idiom, or the use of a phrase or word in a figurative way as opposed to a literal way. To call a person a fifth columnist is not to imply that he or she is somehow related to the events in 1936 or actually makes a column of some sort. Rather, it implies that his or her actions or ideologies are similar to those in the original context. The term has come to refer to anyone who is a member of a subversive organization or otherwise actively seeks to undermine a larger power.