The Collaborative International Dictionary of English v.0.48:

Pillion \Pil"lion\, n. [Ir. pillin, pilliun (akin to Gael.
   pillean, pillin), fr. Ir. & Gael. pill, peall, a skin or
   hide, prob. fr. L. pellis. See Pell, n., Fell skin.]
   1. A panel or cushion saddle; the under pad or cushion of
      saddle; esp., a pad or cushion put on behind a man's
      saddle, on which a woman may ride.
      [1913 Webster]

            His [a soldier's] shank pillion without stirrups.
                                                  --Spenser.
      [1913 Webster]

   2. A saddle or seat behind the driver's saddle on a
      motorcycle, used by passengers.
      [PJC]

WordNet (r) 3.0 (2006):

pillion
    n 1: a seat behind the rider of a horse or motorbike etc.