Sitting in-between the driver and the passenger in a vehicle or sitting behind the driver on a motorcycle.

A person riding between the driver and passenger in a pickup truck. Generally an uncomfortable position in the cab, hence the name.

The person forced to sit in the middle of the back seat or in between the two front seats in a two seater in a car. Usually the smallest person or only female in the group will have to ride 'bitch', so that the guys don't touch each other (irrational male homophobia)

the passenger (usually female) on the back of a motorcycle.