I believe you are referring to a monowheel. A monowheel is like a unicycle in that it has one wheel, however instead of riding above the wheel, a monowheel rider sits inside the wheel.

Monowheels are more stable than unicycles because of their lower center of gravity. Unfortunately they are more difficult to steer, have a very limited carrying capacity, and slowing down tends to cause the rider to rock back and forth in the wheel (called "gerbiling").