Definition from Nolo’s Plain-English Law Dictionary
An attempt to stop others from purchasing products from, performing services for, or otherwise doing business with a company that does business with another company that is in the midst of a labor dispute. For example, if a grocery chain's clerks are on strike, and their union discourages a delivery drivers' union from moving the products of the chain's largest food supply companies, that would be a secondary boycott. The purpose of the secondary boycott is typically to exert indirect pressure on the employer to resolve the labor dispute by causing its business connections to suffer as a result of the dispute. Secondary boycotts are illegal under the National Labor Relations Act.