Horsepower, a unit of power. Power is the rate at which work is done. One horse-power (abbreviation, HP or hp) equals 550 foot-pounds of work per second. (A foot-pound is the amount of work done by one pound of force exerted through a distance of one foot.) The metric unit is the watt; one horsepower equals 745.7 watts.

Originally, horsepower meant the ability of a horse to perform heavy tasks such as turning a mill wheel or drawing a load. James Watt (1736-1819), inventor of the first efficient steam engine, chose horsepower as a standard to which the power of an engine could be meaningfully compared. Watt's measurements of horsepower were widely accepted, even though the measurements applied only to very strong horses working for brief periods. Although the horse is no longer an important source of power, Watt's standard is retained for rating engines, turbines, electric motors, windmills, and water-power devices.

Indicated horsepower is the power an engine would develop if it worked without frictional losses.

Brake, or shaft, horsepower (also called delivered horsepower) is the actual power output of an engine. Mechanical efficiency of an engine is the ratio between brake horsepower and indicated horsepower, and friction horsepower is the difference between indicated horsepower and brake horsepower.

Water horsepower is a measure of water power. The falling of 33,000 pounds (14,969 kg) of water over a distance of one foot (30 cm) in one minute produces one horsepower.

Thrust horsepower is a measure of the power of a jet or rocket engine. It is also a measure of the rotational power that an engine-driven propeller converts into thrust. (Thrust is the forward force of propulsion.)

Taxable horsepower is calculated by state and local governments for the purpose of assessing taxes on automobiles and other machines. Taxable horsepower, usually based on a formula using the diameter and number of cylinders, is generally much lower than brake horsepower.

An average person while running can develop about one horsepower for a brief period and about one-seventh horsepower for sustained periods. Small motors in household appliances are rated at about one-third to three-fourths horsepower.