ancien regime vs authorities

ancien regime and authorities both are nouns.

Word	Noun	Adjective	Verb	Adverb
ancien regime	Yes	No	No	No
authorities	Yes	No	No	No

As nouns, authorities is a hypernym of ancien regime; that is, authorities is a word with a broader meaning than ancien regime:

ancien regime: a political and social system that no longer governs (especially the system that existed in France before the French Revolution)
authorities: the organization that is the governing authority of a political unit

Other hypernyms of ancien regime include government, regime.

ancien regime (noun)	authorities (noun)
a political and social system that no longer governs (especially the system that existed in France before the French Revolution)	the organization that is the governing authority of a political unit