Use 'pillion' when talking about motorcycles to specify the rear seat, especially in transportation contexts. He fitted a new leather pillion for his bike.
Pro Tip 2/3
Not for Drivers
'Pillion' always refers to the passenger, never the driver, which can help avoid confusion. As a pillion, you must hold onto the driver or the grab rails.
Pro Tip 3/3
Safety Gear Implied
Discussing a pillion often suggests the need for safety gear like helmets for those riding in that position. She grabbed her helmet and took her position on the pillion.