The Collaborative International Dictionary of English v.0.48:

Merchantman \Mer"chant*man\, n.; pl. Merchantmen.
   [1913 Webster]
   1. A merchant. [Obs.] --Matt. xiii. 45.
      [1913 Webster]

   2. A trading vessel; a ship employed in the transportation of
      goods, as, distinguished from a man-of-war.
      [1913 Webster]

WordNet (r) 3.0 (2006):

merchantman
    n 1: a cargo ship; "they did much of their overseas trade in
         foreign bottoms" [syn: bottom, freighter,
         merchantman, merchant ship]

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

MERCHANTMAN. A ship or vessel employed in a merchant's service. This term is
used in opposition to a ship of war.