The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

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
    adv 1: one behind the other; "ride tandem on a bicycle built for
           two"; "riding horses down the path in tandem" [syn:
           tandem, in tandem]
    n 1: an arrangement of two or more objects or persons one behind
         another
    2: a bicycle with two sets of pedals and two seats [syn:
       bicycle-built-for-two, tandem bicycle, tandem]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "tandem":
   back to back, behind, behind the scenes, equipage, four-in-hand,
   in back of, in the background, in the rear, pair, randem, rig,
   span, spike, spike team, team, three-up, turnout, unicorn