Students' Union Definition

A student organization present in many colleges, universities, and high schools. In higher education, the students' union is often accorded its own building on the campus, dedicated to social, organizational activities, and representation.

The building where the union provides such services.

A student organization to provide services to students, especially of higher education (in this sense only US English, and then almost always "student union")

The building where the union provides such services. (in this sense only US English, and then almost always "student union")