Second Empire vs empire

Second Empire and empire both are nouns.

Word	Noun	Adjective	Verb	Adverb
Second Empire	Yes	No	No	No
empire	Yes	No	No	No

As nouns, empire is a hypernym of Second Empire; that is, empire is a word with a broader meaning than Second Empire:

Second Empire: the imperial government of Napoleon III in France from 1852-1870
empire: a monarchy with an emperor as head of state

Second Empire (noun)	empire (noun)
the imperial government of Napoleon III in France from 1852-1870	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