pirate vs plunderer

pirate and plunderer both are nouns.

Word	Noun	Adjective	Verb	Adverb
pirate	Yes	No	Yes	No
plunderer	Yes	No	No	No

As nouns, plunderer is a hypernym of pirate; that is, plunderer is a word with a broader meaning than pirate:

pirate: someone who robs at sea or plunders the land from the sea without having a commission from any sovereign nation
plunderer: someone who takes spoils or plunder (as in war)

Other hypernyms of pirate include despoiler, freebooter, looter, pillager, raider, spoiler.

pirate (noun)	plunderer (noun)
a ship that is manned by pirates	someone who takes spoils or plunder (as in war)
someone who robs at sea or plunders the land from the sea without having a commission from any sovereign nation
someone who uses another person's words or ideas as if they were his own

pirate (verb)	plunderer (verb)
take arbitrarily or by force
copy illegally; of published material