mediator vs negotiator

mediator and negotiator both are nouns.

Word	Noun	Adjective	Verb	Adverb
mediator	Yes	No	No	No
negotiator	Yes	No	No	No

As nouns, negotiator is a hypernym of mediator; that is, negotiator is a word with a broader meaning than mediator:

mediator: a negotiator who acts as a link between parties
negotiator: someone who negotiates (confers with others in order to reach a settlement)

Other hypernyms of mediator include negotiant, treater.

mediator (noun)	negotiator (noun)
a negotiator who acts as a link between parties	someone who negotiates (confers with others in order to reach a settlement)