A monowheel is a one-wheeled single-track vehicle similar to a unicycle. However, instead of sitting above the wheel, the rider sits either within it or next to it. The wheel is a ring, usually driven by smaller wheels pressing against its inner rim.

Hand-cranked and pedal-powered monowheels were built in the late 19th century; most built in the 20th century have been motorized.

Via Wikipedia.