An agency shop is a place of employment in which full membership in a union is not required, but the union acts as an agent for the employees. Both union members and non-union members must pay periodic union dues and initiation fees whether they join or not since it is presumed that any collective bargaining on the part of the union will benefit both union members and non-union members.
See, e.g. Seidemann v. Bowen, 499 F.3d 119 (2007)
[Last updated in June of 2022 by the Wex Definitions Team]