intermediary

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Existing or occurring between; intermediate.
• adjective Acting as a mediator or an agent between persons or things.
• noun One that acts as a mediator.
• noun One that acts as an agent between persons or things; a means.
• noun An intermediate state or stage.

from The Century Dictionary.

• noun Same as nerve of Wrisberg (which see, under nerve).
• Being or occurring between; having an intermediate position or action: as, an intermediary process.
• noun One who or that which interposes or is intermediate; an intermediate agent; a go-between.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who, or that which, is intermediate; an interagent; a go-between; a mediator.
• adjective Lying, coming, or done, between; intermediate.
• adjective (Surg.) an amputation for injury, performed after inflammation has set in.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Intermediate.
• noun An agent acting as a mediator between sides that may disagree.
• noun One or several stages of an event which occurs after the start and before the end.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a negotiator who acts as a link between parties