What does "seniority" mean in labor unions?

Seniority, in the context of labor unions, refers to a system that gives preference to workers based on their length of service within the organization or workplace. This principle is often utilized in determining various employment aspects, such as promotions, layoffs, and choice of shifts, where longer-serving employees may receive advantages due to their commitment and experience. By prioritizing seniority, labor unions aim to create a fair and predictable framework for employee treatment, fostering loyalty and stability within the workforce.

The focus on length of service distinguishes this concept from other options, which either center on educational qualifications, age, or innovation, none of which typically govern the seniority system in labor practices.