sovereign vs emperor

sovereign and emperor both are nouns.

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

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

sovereign (noun)	emperor (noun)
a nation's ruler or head of state usually by hereditary right	large richly colored butterfly
	large moth of temperate forests of Eurasia having heavily scaled transparent wings
	red table grape of California
	the male ruler of an empire

sovereign (adjective)	emperor (adjective)
greatest in status or authority or power
(of political bodies) not controlled by outside forces