The power of one horse, which has been a metric for internal combustion engines 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, One Horse
In the late 18th century, Scottish inventor James Watt invented the horsepower measurement as a marketing tool to sell his steam engines to drain mines. What miners really wanted to know was how many horses his engine would replace, so Watt came up with the measurement that one horsepower was equivalent to lifting 550 pounds one foot in one second. Whether every horse could perform that work was irrelevant because Watt invented a metric prospective buyers could relate to.
Horsepower is sometimes just a general measurement of performance; for example, "that engine has a lot of horsepower." See
computer horsepower.