The Collaborative International Dictionary of English v.0.48:

Negotiator \Ne*go"ti*a`tor\, n. [L.: cf. F. n['e]gociateur.]
   One who negotiates; a person who treats with others, either
   as principal or agent, in respect to purchase and sale, or
   public or private compacts.
   [1913 Webster]

WordNet (r) 3.0 (2006):

negotiator
    n 1: someone who negotiates (confers with others in order to
         reach a settlement) [syn: negotiator, negotiant,
         treater]