Solidarity action (also known as secondary action, a secondary boycott) is an action taken by an uninvolved third party to assist one of the primary parties to a dispute. The most commonly encountered form is industrial action by a trade union in support of a strike initiated by workers in a separate corporation, but often the same enterprise, group of companies, or connected firm. This latter type of action is also known as a solidarity strike, or a sympathy strike). Employers can also participate in solidarity action, for example by blacklisting (refusing to hire) employees who have been dismissed by another employer for having taken industrial action. A consumer boycott – refusal to buy the products of one of the participants (a company or even a state) – is another well-known form of solidarity action.
In Australia, Latvia, Luxembourg, the United States, and the United Kingdom, solidarity industrial action is theoretically illegal, and strikes can only be against the contractual employer. Germany, Italy and Spain have restrictions in place that restrict the circumstances in which solidarity action can take place (see European labour law).