monarch vs emperor

monarch and emperor both are nouns.

Word	Noun	Adjective	Verb	Adverb
monarch	Yes	No	No	No
emperor	Yes	No	No	No

As nouns, emperor is a hyponym of monarch; that is, emperor is a word with a more specific, narrower meaning than monarch:

monarch (noun)	emperor (noun)
large migratory American butterfly having deep orange wings with black and white markings; the larvae feed on milkweed	large richly colored butterfly
a nation's ruler or head of state usually by hereditary right	large moth of temperate forests of Eurasia having heavily scaled transparent wings
	red table grape of California
	the male ruler of an empire