The Collaborative International Dictionary of English v.0.48:

Middleman \Mid"dle*man\, n.; pl. Middlemen.
   [1913 Webster]
   1. An agent between two parties; a broker; a go-between; any
      dealer between the producer and the consumer; in Ireland,
      one who takes land of the proprietors in large tracts, and
      then rents it out in small portions to the peasantry.
      [1913 Webster]

   2. A person of intermediate rank; a commoner.
      [1913 Webster]

   3. (Mil.) The man who occupies a central position in a file
      of soldiers.
      [1913 Webster]

WordNet (r) 3.0 (2006):

middleman
    n 1: someone who buys large quantities of goods and resells to
         merchants rather than to the ultimate customers [syn:
         jobber, middleman, wholesaler]
    2: the performer in the middle of a minstrel line who engages
       the others in talk [syn: interlocutor, middleman]
    3: a person who is in a position to give you special assistance;
       "he used his business contacts to get an introduction to the
       governor" [syn: contact, middleman]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "middleman":
   agent, arbitrator, broker, chandler, connection, contact, dealer,
   deputy, distributor, entrepreneur, front, front man, go-between,
   importer, interagent, interceder, intercessor, intermediary,
   intermediate, intermediate agent, intermediator, intermedium,
   internuncio, interpleader, intervener, interventionist,
   interventor, jobber, liaison, link, marketer, mediary, mediator,
   medium, merchandiser, merchant, monger, mouthpiece, negotiant,
   negotiator, negotiatress, negotiatrix, ombudsman, regrater,
   retail dealer, retail merchant, retailer, shopkeeper, spokesman,
   spokeswoman, storekeeper, tie, trader, tradesman, tradeswoman,
   trafficker, wholesaler



Bouvier's Law Dictionary, Revised 6th Ed (1856):

MIDDLEMAN contracts. A person who is employed both by the seller and
purchaser of goods, or by the purchaser alone, to receive them into his
possession, for the purpose of doing something in or about them; as, if
goods be delivered from a ship by the seller, to a wharfinger, to be by him
forwarded to the purchaser, who has been appointed by the latter to receive
them; or if goods be sent to a packer, for and by orders of the vendee, the
packer is to be considered as a middleman.
     2. The goods in both, these cases will be considered in transitu,
provided the purchaser has not used the wharfinger's or the packer's
warehouse as his own, an have an ulterior place of delivery in view. 3 B. &
P. l27, 469; 4 Esp. R. 82; 2 B. & P. 457; 1 Campb. 282; 1 Atk. 245; 1 H. Bl.
364; 3 East, R. 93; Whit. on Trans. 195.
     3. By middleman is also understood one who has been employed as an
agent by a principal, and who has employed a subagent under him by authority
of the principal, either express or implied. He is not in general liable for
the wrongful acts of the sub-agent, the principal being alone responsible. 3
Campb. N. P. Cas. 4; 6 T. R. 411; 14 East, 65.