What is the meaning of Pillion?

A pad behind the saddle of a horse for a second rider.

A similar second saddle on a motorcycle for a passenger.

The person riding in the pillion.

The cushion of a saddle.

Riding behind the driving rider, as when positioned on the rump of a mount.

To ride on a pillion.

To put a pillion on a horse.

Source: wiktionary.org