plaintiff vs petitioner

plaintiff and petitioner both are nouns.

Word	Noun	Adjective	Verb	Adverb
plaintiff	Yes	No	No	No
petitioner	Yes	No	No	No

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

plaintiff: a person who brings an action in a court of law
petitioner: someone who petitions a court for redress of a grievance or recovery of a right

Other hyponyms of plaintiff include suer.

plaintiff (noun)	petitioner (noun)
a person who brings an action in a court of law	one praying humbly for something
	someone who petitions a court for redress of a grievance or recovery of a right