The Collaborative International Dictionary of English v.0.48:

Broker \Bro"ker\ (br[=o]"k[~e]r), n. [OE. brocour, from a word
   akin to broken, bruken, to use, enjoy, possess, digest, fr.
   AS. br[=u]can to use, enjoy; cf. Fries. broker, F.
   brocanteur. See Brook, v. t.]
   1. One who transacts business for another; an agent.
      [1913 Webster]

   2. (Law) An agent employed to effect bargains and contracts,
      as a middleman or negotiator, between other persons, for a
      compensation commonly called brokerage. He takes no
      possession, as broker, of the subject matter of the
      negotiation. He generally contracts in the names of those
      who employ him, and not in his own. --Story.
      [1913 Webster]

   3. A dealer in money, notes, bills of exchange, etc.
      [1913 Webster]

   4. A dealer in secondhand goods. [Eng.]
      [1913 Webster]

   5. A pimp or procurer. [Obs.] --Shak.
      [1913 Webster]

   Bill broker, one who buys and sells notes and bills of
      exchange.

   Curbstone broker or Street broker, an operator in stocks
      (not a member of the Stock Exchange) who executes orders
      by running from office to office, or by transactions on
      the street. [U.S.]

   Exchange broker, one who buys and sells uncurrent money,
      and deals in exchanges relating to money.

   Insurance broker, one who is agent in procuring insurance
      on vessels, or against fire.

   Pawn broker. See Pawnbroker.

   Real estate broker, one who buys and sells lands, and
      negotiates loans, etc., upon mortgage.

   Ship broker, one who acts as agent in buying and selling
      ships, procuring freight, etc.

   Stock broker. See Stockbroker.
      [1913 Webster]

WordNet (r) 3.0 (2006):

broker
    n 1: a businessman who buys or sells for another in exchange for
         a commission [syn: agent, factor, broker]
    v 1: act as a broker

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "broker":
   Realtor, Wall Streeter, agent, arbitrator, bill broker, bond crowd,
   cambist, connection, contact, cotton broker, curb broker, dealer,
   diamond broker, discount broker, distributor, estate agent,
   floor broker, floor trader, floorman, furniture broker, go-between,
   grain broker, hotel broker, insurance broker, interagent,
   interceder, intercessor, intermediary, intermediate, intermediator,
   intermedium, internuncio, interpleader, jobber, land broker,
   liaison, link, mediary, mediator, medium, middleman, money broker,
   money changer, mortgage broker, negotiant, negotiator, note broker,
   odd-lot dealer, pawnbroker, pit man, real estate agent,
   real estate broker, registered representative, sharebroker,
   ship broker, specialist, stock dealer, stock-exchange broker,
   stockbroker, stockjobber, tie, two-dollar broker, wholesaler



The Free On-line Dictionary of Computing (30 December 2018):

broker

   object request broker