bro·ker   [broh-ker]
A person who functions as an intermediary between two or more parties in negotiating agreements, bargains, or the like.
Origin: Middle English, from Anglo-Norman brocour- From Dictionary.com:

Sometimes it is presumed that a real estate broker is solely someone who finds a buyer for a business or a tenant for a retail store. In truth, the broker’s job is only partially done once that’s been accomplished, as the consummation of a transaction is anything but assured at that point. From the definition then, it can be inferred that a broker’s job requires the ability to successfully navigate between two parties of often different backgrounds, experience, education, and communication styles, as well as in their understanding of the various points that require agreement in order to complete a lease or sale. At any point, misunderstandings can (and often do) develop that can needlessly sabotage an intention by both parties to do business. So it is, in fact, the effective application of all of the aforementioned skills that entitles a “broker” to a commission.