plenipotentiary vs diplomatist

plenipotentiary and diplomatist both are nouns.

Word	Noun	Adjective	Verb	Adverb
plenipotentiary	Yes	No	No	No
diplomatist	Yes	No	No	No

As nouns, diplomatist is a hypernym of plenipotentiary; that is, diplomatist is a word with a broader meaning than plenipotentiary:

plenipotentiary: a diplomat who is fully authorized to represent his or her government
diplomatist: an official engaged in international negotiations

Other hypernyms of plenipotentiary include diplomat.

plenipotentiary (noun)	diplomatist (noun)
a diplomat who is fully authorized to represent his or her government	an official engaged in international negotiations