a one-wheeled device attached to the side of a motorcycle, scooter, or bicycle
A sidecar is a small, one-wheeled vehicle that attaches to the side of a motorcycle. This allows one or two additional passengers to be carried in the vehicle. The sidecar's wheel is not in line with any of the motorcycle's wheels.
German troops used many motorcycles with sidecars in World War II, from the BMW and Zündapp brands.