A pump jack, nodding donkey, pumping unit, or horsehead pump is the overground drive for a reciprocating piston pump installed in a borehole. It is used to mechanically lift liquid out of the well if there is not enough bottom hole pressure for the liquid to flow all the way to the surface.

A pumpjack is commonly powered by an electric motor, but older, less producing wells use propane as an alternative to the high price of installing a large power grid. Some of these wells can even use the natural gas from the casing as fuel, and the well can be completely self sufficient. Diesel engines are also used with weekly maintenance trucks supplying fuel in large tanks. These motors and engines are housed inside a shack to prevent rain and snow from getting on the engine and causing it to fail.

The engine of the pumpjack runs a set of pulleys to the transmission which in turn drives a pair of cranks, generally with counter weights on them to assist the motor in lifting the heavy string of rods. The cranks in turn raise and lower one end of an I beam which is free to move on an A frame. On the other end of the beam, there is a curved metal box called a Horse Head or Donkeys Head by their appearance. A metal cable (or occasionally, fiberglass) called a bridle, connects the horse head to the polished rod. The bridle follows the curve of the horse head as it lowers and raises to create a completely vertical stroke.

Depending on the size of the pump, it generally produces 5 to 40 litres of a crude oil-water mixture (called emulsion) at each stroke. The size of the pump is also determined by the depth and weight of the oil to be removed, with deeper extraction requiring more power to move the heavier lengths of polish rod.

A pump jack converts the rotary mechanism of the motor to a vertical reciprocating motion to drive the pump shaft, and is exhibited in the characteristic nodding motion. The engineering term for this type of mechanism is a walking beam. It was often employed in stationary and marine steam engine designs in the 1700s and 1800s.

This system is also commonly referred to as a beam pump, rod pump, grasshopper, thirsty bird or jack pump. This type of arrangement is commonly used in onshore applications for relatively low-production oil wells.

Pumpjacks are common in many oil-rich areas, dotting the countryside and occasionally serving as local landmarks.