closed shop

plural closed shops

1. A business or industrial establishment whose employees are required to be labor union members as a precondition to employment.

   [...] about 90 per cent of the locomotive staff belong to one trade union and 10 per cent to another smaller one or to none, for there is no such thing in Switzerland as a "closed shop".

   1963 January, ""Mercury"", “Driver training and single-manning in Switzerland”, in Modern Railways, page 42

2. historicalcomputingA facility that runs programs on behalf of others, giving them no oversight over execution but merely returning the output at the end.