A labor union is an organization that develops where a workforce, who have a common trade or work at the same establishment, band together in order to take advantage of the increased power of collective bargaining.
Chief Miles O'Brien proudly claimed that his ancestor, Sean Aloysius O'Brien, led his union into the eleven-month Anthracite strike of 1902, before being killed one week before the strike's end. His body was found in the Allegheny River, riddled with bullets. (DS9: "Bar Association")
Rom formed a union in late 2372, after Quark became even more ruthless to his staff. Brunt and the Ferengi Commerce Authority became aware of it and took steps to destroy it. To avoid further trouble with the FCA, Quark gave all of his staff an increase in wages in exchange for Rom dissolving the union. (DS9: "Bar Association")
Rom cited the union as having no effect on Ferengi attitudes to him, although his brother caustically told him "they didn't want anything to do with you before you formed the union..." (DS9: "Body Parts")