If a factory, shop, or other business is a closed shop, the employees must be members of a particular trade union.
[business]
...the trade union which they are required to join under the closed shop agreement.
closed shop in British English
noun
(formerly) an industrial establishment in which there exists a contract between a trade union and the employer permitting the employment of the union's members only
Compare open shop, union shop
closed shop in American English
US
1.
a factory, business, etc. operating under a contractual arrangement between a labor union and the employer by which only members of the union may be employed
2.
this arrangement
Examples of 'closed shop' in a sentence
closed shop
Several bright blue soda machines on the sidewalk were against the brick wall of a closed shop.