open shop

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A place or policy of employment that does not permit discrimination against employees based on membership or nonmembership in a labor union.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A place of employment in which employment is in no way conditioned on membership in a union, though a union may be the exclusive bargaining agent of the employees.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a company whose workers are hired without regard to their membership in a labor union