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 for one person.
Horsepower is sometimes stated as a general measurement of performance; for example, "that has a lot of horsepower" refers to a powerful engine of some type. See
computer horsepower.