monarchy vs empire

monarchy and empire both are nouns.

Word	Noun	Adjective	Verb	Adverb
monarchy	Yes	No	No	No
empire	Yes	No	No	No

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

monarchy: an autocracy governed by a monarch who usually inherits the authority
empire: a monarchy with an emperor as head of state

Other hyponyms of monarchy include parliamentary monarchy, kingdom.

monarchy (noun)	empire (noun)
an autocracy governed by a monarch who usually inherits the authority	a monarchy with an emperor as head of state
	a group of countries under a single authority
	a group of diverse companies under common ownership and run as a single organization
	the domain ruled by an emperor or empress; the region over which imperial dominion is exercised