agency shop

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

• noun An establishment in which a union represents all employees regardless of union membership and requires that nonmembers pay union dues or fees.

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

• noun A place of employment in which a union receives dues or the equivalent for all employees excluding management, and bargains for all such employees whether or not they are members of the union.