What is Union Shop?

Legal Definition
A union shop is a form of a union security clause. Under this, the employer agrees to either only hire labor union members or to require that any new employees who are not already union members become members within a certain amount of time. Use of the union shop varies widely from nation to nation, depending on the level of protection given trade unions in general.
-- Wikipedia
Legal Definition
Clause of union agreements requiring all employees to be members within 30 days of starting work. Also known as association shop.