The union at the University of Wisconsin-Madison. There are actually two union buildngs: Memorial Union and Union South. Quite a lot of organizations at UW are associated with the Union in some way (if nothing else, they use Union space), including the Wisconsin Hoofers. Mini Courses are also run through the union.
The Wisconsin Union was founded in 1914, and was one of the seven original members of the ACUI. The responsible body for the governance of the Union is the Wisconsin Union Directorate.