According to Barron's Law Dictionary, a "Yellow Dog Contract" is defined as follows: "An employment contract expressly prohibiting the named employee from joining labor unions under pain of dismissal. Typically, a yellow dog contract will contain the following three provisions:
(1) A representation by the employee that he or she is not a member of a labor union;
(2) A promise by the employee not to join a labor union;
(3) A promise by the employee to quit his or her job upon joining a labor union.
Yellow dog contracts are not enforceable in federal courts, and are prohibited under the laws or constitutions of most states.