A monowheel (monocycle, monowheel) is a one-wheeled vehicle, but there the similarity to a unicycle ends. The rider sits within the circumference of the wheel which is of large diameter and is driven by small wheels on the inside of the rim.
Think of a hamster wheel.
Most mono wheels are single-seaters, although two-seates have been built where the riders sit on either side of the wheel.
In the late 19th century, the mono wheels were pedal driven, whereas most 20th century mono wheels are powered.
Sources: Wikipedia.nl, et al.