queen regnant vs female monarch

queen regnant and female monarch both are nouns.

Word	Noun	Adjective	Verb	Adverb
queen regnant	Yes	No	No	No
female monarch	Yes	No	No	No

As nouns, queen regnant and female monarch are synonyms defined as:

Other synonyms of queen regnant include queen.

queen regnant (noun)	female monarch (noun)
a female sovereign ruler	a female sovereign ruler