IRS vs authority

IRS and authority both are nouns.

Word	Noun	Adjective	Verb	Adverb
IRS	Yes	No	No	No
authority	Yes	No	No	No

As nouns, authority is a hypernym of IRS; that is, authority is a word with a broader meaning than IRS:

Other hypernyms of IRS include agency, bureau, federal agency, government agency, office.

IRS (noun)	authority (noun)
the bureau of the Treasury Department responsible for tax collections	the power or right to give orders or make decisions
	official permission or approval
	freedom from doubt; belief in yourself and your abilities
	an authoritative written work
	an administrative unit of government
	an expert whose views are taken as definitive
	(usually plural) persons who exercise (administrative) control over others