Horsepower (hp) is a unit of measurement of power (the rate at which work is done). There are many different standards and types of horsepower. Two common definitions being used today are the mechanical horsepower (or imperial horsepower), which is approximately 746 watts, and the metric horsepower, which is approximately 735.5 watts.
The term was adopted in the late 18th century by Scottish engineer James Watt to compare the output of steam engines with the power of draft horses. It was later expanded to include the output power of other types of piston engines, as well as turbines, electric motors and other machinery