Dictionary:

mediator

  (mē'dē-ā'tər)

1. One that mediates, especially one that reconciles differences between disputants.
2. Physiology. A substance or structure that mediates a specific response in a bodily tissue.

noun

Someone who acts as an intermediate agent in a transaction or helps to resolve differences: broker, go-between, interceder, intercessor, intermediary, intermediate, intermediator, middleman. See means.

Definition: person who negotiates agreement
Antonyms: arguer, fighter

The noun has one meaning:

Meaning #1: a negotiator who acts as a link between parties
  Synonyms: go-between, intermediator, intermediary