labor union

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An organization of workers formed for the purpose of serving the members' interests with respect to wages and working conditions.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun North America A continuous association of wage-earners for the purpose of maintaining or improving the conditions of their employment; a trade union.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an organization of employees formed to bargain with the employer