The power of one horse, which has been a measurement of internal combustion engine power for more than a hundred years. When automobiles emerged in the late 19th century, their performance was measured against horses, the only other mode of self-propelled transportation at that time.
Technically, More than One Horse
By observing the effort of a horse turning a mill wheel, one horsepower was computed as equivalent to lifting 33,000 pounds one foot in one minute. In fact, horses could actually do more work.
Horsepower is sometimes just a general measurement of performance; for example, "that engine has a lot of horsepower." See
computer horsepower.