A pillion is a mostly British English term for a secondary pad, cushion, or seat behind the main seat or saddle on a horse, motorcycle, bicycle or moped. A passenger in this seat is said to "ride pillion." The word is derived from the Scottish Gaelic for a "little rug," pillean, which is itself from the Latin pellis for "animal skin." One or more pelts would often have been used as a secondary seat on horseback; the usage has carried over to motorcycles.
Following its retreat from the Battle of Dunkirk (where it is reputed that enough equipment was left behind to equip about eight to ten divisions), the British Army introduced a requirement that all officers up to the rank of colonel should be proficient in the use of the motorcycle, and all officers holding the rank of brigadier were required to be able to ride pillion. These requirements came about as a result of the large number of motor cars that were lost in action. The requirement for riding pillion was quietly dropped as large numbers of jeeps came into service in the middle of the war.
"Riding two up" is a commonly accepted North American phrase for riding with a passenger.
"Riding bitch" is a expression used in the U.S. to denote sitting in between two other people in a car or truck, where there is often a hump in the front or back analogous to the pillion (due in the front to the presence of the transmission housing). "Bitch seat" or "bitch pad" is North American slang for the pillion on a motorcycle, as is "riding bitch" instead of "riding pillion".
To carry a pillion passenger in the United States or United Kingdom, one must hold a full licence for the vehicle and there must also be a proper seat and foot pegs for the passenger. In the UK, a motorcyclist is not allowed to carry more than one pillion passenger. This is due to motorcycles not being designed to carry more than one pillion passenger.
In Australia, the vehicle operator must have held their licence (not including a learner's permit) for a minimum of one year before being legally allowed to carry a passenger where physically possible, following the upgrade to the required licence class. For example, in New South Wales, you must carry a Provisional 2 (Green) licence before being allowed to carry a pillion passenger.