ambassador vs diplomatist

ambassador and diplomatist both are nouns.

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

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

ambassador: a diplomat of the highest rank; accredited as representative from one country to another
diplomatist: an official engaged in international negotiations

Other hypernyms of ambassador include diplomat.

ambassador (noun)	diplomatist (noun)
a diplomat of the highest rank; accredited as representative from one country to another	an official engaged in international negotiations
an informal representative