The Collaborative International Dictionary of English v.0.48:

Trainer \Train"er\, n.
   1. One who trains; an instructor; especially, one who trains
      or prepares men, horses, etc., for exercises requiring
      physical agility and strength.
      [1913 Webster]

   2. A militiaman when called out for exercise or discipline.
      [U. S.] --Bartlett.
      [1913 Webster]

WordNet (r) 3.0 (2006):

trainer
    n 1: one who trains other persons or animals
    2: simulator consisting of a machine on the ground that
       simulates the conditions of flying a plane [syn: flight
       simulator, trainer]