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.

Passenger seat or sometimes the passenger is described as "riding pillion" (mostly a U.K. term).

a seat behind the rider of a horse or motorbike etc.

a cushion, pad, or carrier, fastened on the back of a womans' saddle, usually for a light female passenger

( adjective) A passenger on a motorcycle.

A small cushion designed for carrying a passenger mounted behind a solo saddle.

A pillion is the back seat on a motorcycle.

A pillion is a secondary pad, cushion, or seat behind the main seat or saddle on a horse, motorcycle, or moped. A passenger in this seat is said to "ride pillion" or may themselves be referred to as a "pillion." The word is derived from the Gaelic for "little rug," pillean, which is itself from the Latin pellis for "animal skin."