A lockout occurs when:

the union forces another union to stop handling the products of the primary employer until a dispute is settled.

an agreement is reached where the union gets firms that buy products from the employer that is being struck to agree not to buy goods from that employer until that dispute is settled.

the employer refuses to permit employees to work until a labor dispute is settled.

a picket is placed around a firm that agrees to take a partially finished product and finish it with the firm against whom the union is having a labor dispute.