1. a seat behind the rider of a horse or motorbike etc.
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.
His [a soldier's] shank pillion without stirrups.
2. A saddle or seat behind the driver's saddle on a
motorcycle, used by passengers.
WordNet (r) 3.0 (2006):
n 1: a seat behind the rider of a horse or motorbike etc.