Generally, Stool pigeon is an informant who provides privileged information about a person or organization to an agency without the consent of that person or organization. When this is done for a law enforcement agency such person is called a criminal informant.
In relation to labor, stool pigeon is a person hired by an employer to infiltrate the union and report on its activities. They can also be labor organization informers who are hired by corporations to monitor or control labor organizations and their activities. Such persons may be professionals or recruits from the workforce. They may be willingly doing it or may be tricked into informing about their co-workers' unionization effort.