A variation of a waterwheel in which the waterwheel was powered by water coming over the top side of the wheel, as opposed to an undershoot waterwheel where the water turns the wheel from the bottom.
The first mills to employ overshoot waterwheels are found as early as the 13th century of the common era. They were 50-60% efficient. Many of these wheels had evidence that they were used in conjuction with the a cam to allow the conversion of rotary motion into reciprocating (alternating on one axis) motion.