legal separation vs fiat

legal separation and fiat both are nouns.

Word	Noun	Adjective	Verb	Adverb
legal separation	Yes	No	No	No
fiat	Yes	No	No	No

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

legal separation: a judicial decree regulating the rights and responsibilities of a married couple living apart
fiat: a legally binding command or decision entered on the court record (as if issued by a court or judge)

Other hypernyms of legal separation include decree, edict, order, rescript.

legal separation (noun)	fiat (noun)
(law) the cessation of cohabitation of man and wife (either by mutual agreement or under a court order)	a legally binding command or decision entered on the court record (as if issued by a court or judge)
a judicial decree regulating the rights and responsibilities of a married couple living apart