Definitionen
1.NounA 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.
2.NounThe building where the union provides such services.
3.NounA student organization to provide services to students, especially of higher education (in this sense only US English, and then almost always "student union")
4.NounThe building where the union provides such services. (in this sense only US English, and then almost always "student union")