student union vs edifice

student union and edifice both are nouns.

Word	Noun	Adjective	Verb	Adverb
student union	Yes	No	No	No
edifice	Yes	No	No	No

As nouns, edifice is a hypernym of student union; that is, edifice is a word with a broader meaning than student union:

student union: a building on a college campus dedicated to social and organizational activities of the student body
edifice: a structure that has a roof and walls and stands more or less permanently in one place

Other hypernyms of student union include building.

student union (noun)	edifice (noun)
a building on a college campus dedicated to social and organizational activities of the student body	a structure that has a roof and walls and stands more or less permanently in one place