A negotiator was a person who engaged in the negotiation of treaties and similar arrangements.
According to Mendoza in 2366, Devinoni Ral was the "best hired gun in the business", meaning the best negotiator for different clients. (TNG: "The Price")
In a simulation created by Barash in 2367, Tomalak called Captain Riker a negotiator "even the Ferengi should avoid." (TNG: "Future Imperfect")
In 2369, Krax told Quark that his imposing features would be enough to convince the negotiators of Stakoron II to give in to Ferengi demands. (DS9: "The Nagus")
In 2370, Quark was appointed the Ferengi chief negotiator in the Gamma Quadrant by Grand Nagus Zek. (DS9: "Rules of Acquisition")
In 2374, after Remata'Klan had advised Benjamin Sisko not to accept the terms he himself had just delivered, Sisko replied that apparently the Jem'Hadar was not a very good negotiator. (DS9: "Rocks and Shoals")
When deliberating how to free Ishka from the Dominion later that year, Rom reminded his brother that Ferengi were not commandoes, but negotiators. (DS9: "The Magnificent Ferengi")
In 2378, Chakotay added "trade negotiator" to the list of positions Neelix had aboard USS Voyager, in addition to ambassador or morale officer. (VOY: "Homestead")