Source : Webster's Revised Unabridged Dictionary (1913)

Tandem \Tan"dem\, n.
   A tandem bicycle or other vehicle.

Tandem \Tan"dem\, adv. & a. [L. tandem at length (of time only),
   punningly taken as meaning, lengthwise.]
   One after another; -- said especially of horses harnessed and
   driven one before another, instead of abreast.

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
     n : a bicycle with two sets of pedals and two seats [syn: {bicycle-built-for-two},
          {tandem bicycle}]
     adv : one behind the other; "ride tandem on a bicycle built for
           two"; "riding horses down the path in tandem"