king vs male monarch

king and male monarch both are nouns.

Word	Noun	Adjective	Verb	Adverb
king	Yes	No	No	No
male monarch	Yes	No	No	No

As nouns, king and male monarch are synonyms defined as:

Other synonyms of king include Rex.

king (noun)	male monarch (noun)
(chess) the weakest but the most important piece	a male sovereign; ruler of a kingdom
one of the four playing cards in a deck bearing the picture of a king
a checker that has been moved to the opponent's first row where it is promoted to a piece that is free to move either forward or backward
a male sovereign; ruler of a kingdom
a very wealthy or powerful businessman
a male competitor who holds a preeminent position
the husband or widower of a queen
preeminence in a particular category or group or field