Definition of Yellow dog contract
An employment practice by which employer requires employee to sign an agreement promising as condition of employment that he will not join a union and will be discharged if he does join. Such contracts are prohibited by the National Labor Relations Act, the Norris LaGuardia Act, the Railway Labor Act, and as well by the laws of most states. 29 U.S.C.A. § 103.
That's the definition of Yellow dog contract in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.