By the American definition, one horsepower (HP) is the amount of energy required to lift 550 pounds in one foot in one second, disregarding any friction. While the common unit for measuring energy is the Watt, horsepower is a term used to describe the kind of energy that is produced by an internal combustion engine, such as that in a car, motorcycle, or airplane, among many others.
When steam engines were invented in the early 1700s, there was a need to equate the amount of work that they could do in comparison to something to which everyone could relate. Because most people had some sort of background in farming and knew of the usefulness of horses in the fields, steam engine power was related to horses by the term "horsepower". In those days, a steam engine that could produce the same amount of energy as two horses was considered a two-horsepower engine.
By the standard definition of horsepower:
An aircraft’s engine is usually not rated by horsepower, but by the thrust that the motor produces. It is difficult to convert between horsepower and thrust because they are two very different units of measure, but as a general rule of thumb:
Let’s say that a light business jet probably has a total engine thrust of 10,000 pounds, and the aircraft is traveling in level flight at a cruise speed of 300 knots. By the above equation, that aircraft would be producing an equivalent of 9,231 horsepower. By comparison, a Mini Cooper automobile produces approximately 150 horsepower, while the Space Shuttle produces a maximum of almost 37 million horsepower!