In general, power is the amount of work that can be done in a certain amount of time. When looking at an internal combustion engines, such as those found in automobiles and non-jet airplanes, it is the term used to describe the output, or product, of that engine. The unit most often used to describe the power of cars and airplanes is horsepower (hp).

Horsepower is defined as the amount of power that can move a 550-pound object 1 foot in one second of time, or 550 ft x (lbs/s). Another way of thinking about this is that a 1 horsepower (hp) engine can move a 550-pound car 1 foot within one second. So, a 10-horsepower engine can move a 550-pound car 10 feet within one second.

Although horsepower is used to rate automobile and airplane engines in the United States, the watt (or kilowatt--1000 watts) is its metric equivalent and is customarily used for things like electric lines or transformers.