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]