What is the term for an illegal dispute that exerts pressure on a contractor to settle with a union?

The term that refers to an illegal dispute used to pressure a contractor to settle with a union is known as a secondary boycott. This tactic involves a union attempting to persuade others, such as suppliers or customers, to cease doing business with the contractor in order to exert additional pressure for the desired settlement.

Secondary boycotts are considered illegal under certain labor laws because they can disrupt the operations of businesses that are not directly involved in the dispute. It highlights a strategic approach where indirect pressure is applied, contrasting with more direct forms of protest or conflict like strikes or work stoppages.

Other terms mentioned in the options do not accurately describe this type of illegal dispute. For instance, a primary strike occurs when union members stop working to protest their own employer’s practices, while a lockout refers to an employer's action of closing the workplace to prevent employees from working during a dispute. A work stoppage is a more general term for any cessation of work, which could be voluntary or involuntary without the specific intent to pressure through boycotting.