Dictionary.net

(Mech.) A wheel set in motion by moving water, and driving a train of machinery ; it may be either an Undershot, Overshot, or Breast wheel. The undershot wheel is driven by the impulse of the moving water against the float-boards ; in the overshotwheel the water flows from above into buckets, thereby overweighing the wheel on one side and causing it to turn ; in the breast-wheel the water flows into buckets on the lower part of the wheel, and is kept from flowing out of them by a curved trough or breast, within which the buckets move, until they have passed the lowest point.