Source : Webster's Revised Unabridged Dictionary (1913)

Tandem \Tan"dem\, n.
   A team of horses harnessed one before the other. ``He drove
   tandems.'' --Thackeray.

   {Tandem engine}, a compound steam engine having two or more
      steam cylinders in the same axis, close to one another.

   {Tandem bicycle} or {tricycle}, one for two persons in which
      one rider sits before the other.

Source : WordNet®

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