A more thorough explanation:

A defensive lockout is when an employer closes their business and stops employees from working because of a labor dispute. This type of lockout is done to prevent financial harm to the company or to protect a legal right.

For example, if a union threatens to strike and the employer believes that the strike will cause irreparable financial harm to the company, the employer may initiate a defensive lockout to prevent the strike from happening.

It is important to note that defensive lockouts were legal, but the U.S. Supreme Court abolished the distinction between defensive and offensive lockouts in favor of a balancing test. Offensive lockouts, which are called by management to assert economic pressure on workers and gain a bargaining advantage over a union, were illegal before this decision.

Loosely, an employee's refusal to work because the employer unreasonably refuses to abide by an expired employment contract while a new one is being negotiated can also be considered a defensive lockout.

For example, if an employee's contract has expired and the employer refuses to negotiate a new one, the employee may refuse to work until a new contract is agreed upon. This is considered a defensive lockout because the employee is protecting their legal right to a fair contract.