union shop

  1. a factory, business, etc. operating under a contract between the employer and a labor union, which permits the hiring of nonunion workers but requires that all new employees join the union within a specified period
  2. this contract

See union shop in American Heritage Dictionary 4

noun
A business or industrial establishment whose employees are required to be union members or to agree to join the union within a specified time after being hired. Also called closed shop.

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