1.A member of a clandestine group of people who work subversively within a group, organization, or country in order to betray it to an allied outside force.
机译:秘密小组谁一组,组织或国家内以颠覆性的工作人员中的一员将其出卖给盟军外力。
In the US during the Cold War, there was constant fear and suspicion of people supposedly working as fifth columnists to spread the influence of communism from within.