papal bull vs fiat

papal bull and fiat both are nouns.

Word	Noun	Adjective	Verb	Adverb
papal bull	Yes	No	No	No
fiat	Yes	No	No	No

As nouns, fiat is a hypernym of papal bull; that is, fiat is a word with a broader meaning than papal bull:

papal bull: a formal proclamation issued by the pope (usually written in antiquated characters and sealed with a leaden bulla)
fiat: a legally binding command or decision entered on the court record (as if issued by a court or judge)

Other hypernyms of papal bull include decree, edict, order, rescript.

papal bull (noun)	fiat (noun)
a formal proclamation issued by the pope (usually written in antiquated characters and sealed with a leaden bulla)	a legally binding command or decision entered on the court record (as if issued by a court or judge)