The Collaborative International Dictionary of English v.0.48:

Pusher \Push"er\, n.
   1. One who, or that which, pushes.
      [1913 Webster]

   2. One who sells illegal drugs, esp. one who tries to
      convince others to use such drugs.
      [PJC]

   3. (Railroad) a second engine attached to the rear of a train
      to provide extra power for climbing steep grades.
      [PJC]

WordNet (r) 3.0 (2006):

pusher
    n 1: one who intrudes or pushes himself forward [syn: pusher,
         thruster]
    2: an unlicensed dealer in illegal drugs [syn: pusher, drug
       peddler, peddler, drug dealer, drug trafficker]
    3: someone who pushes [syn: pusher, shover]
    4: a sandal attached to the foot by a thong over the toes [syn:
       pusher, zori]
    5: a small vehicle with four wheels in which a baby or child is
       pushed around [syn: baby buggy, baby carriage,
       carriage, perambulator, pram, stroller, go-cart,
       pushchair, pusher]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "pusher":
   Mafioso, black marketeer, bootlegger, dealer, double-prop,
   drug pusher, gray marketeer, moonshiner, multi-prop, piston plane,
   propeller plane, propjet, pusher plane, racketeer, single-prop,
   tractor, turbo-prop, turbo-propeller plane, twin-prop