Source : Webster's Revised Unabridged Dictionary (1913)

Treater \Treat"er\, n.
   One who treats; one who handles, or discourses on, a subject;
   also, one who entertains.

Source : WordNet®

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