The Collaborative International Dictionary of English v.0.48:

Tandem \Tan"dem\, n.
   1. A team of horses harnessed one before the other. "He drove
      tandems." --Thackeray.
      [1913 Webster]

   2. A tandem bicycle or other vehicle.
      [Webster 1913 Suppl.]

   Tandem bicycle or Tandem tricycle, one for two persons in
      which one rider sits before the other.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tandem bicycle
    n 1: a bicycle with two sets of pedals and two seats [syn:
         bicycle-built-for-two, tandem bicycle, tandem]