admiral vs flag officer

admiral and flag officer both are nouns.

Word	Noun	Adjective	Verb	Adverb
admiral	Yes	No	No	No
flag officer	Yes	No	No	No

As nouns, flag officer is a hypernym of admiral; that is, flag officer is a word with a broader meaning than admiral:

admiral: the supreme commander of a fleet; ranks above a vice admiral and below a fleet admiral
flag officer: a senior naval officer above the rank of captain

admiral (noun)	flag officer (noun)
any of several brightly colored butterflies	a senior naval officer above the rank of captain
the supreme commander of a fleet; ranks above a vice admiral and below a fleet admiral