What is Bargaining Unit?

Legal Definition
A bargaining unit, in labor relations, is a group of employees with a clear and identifiable community of interests who are (under U.S. law) represented by a single labor union in collective bargaining and other dealings with management. Examples would be non-management professors, law enforcement professionals, blue-collar workers, clerical and administrative employees, etc. Geographic location as well as the number of facilities included in bargaining units can be at issue during representation cases.

The size of a company does not relate to the size of a bargaining unit. Bargaining units must consist of at least three employees, and must have the support of a majority of employees in the bargaining unit. However, the bargaining unit can be a small portion of a large company where no other employees are members of a union.
-- Wikipedia
Legal Definition
A group of employees who have a clear and identifiable community of interests with one another and that are represented by a single labor union in dealings with management such as collective bargaining. The "appropriateness" of a bargaining unit is determined at the discretion of the Regional Director of the Board.
Legal Definition
A group of employees in one union that work together in collective bargaining.