1.NounAny of a class of machines that used draft horses as a source of power to an output shaft or sheave. Treadmill types and merry-go-round–like types existed.
"In that era, they powered their corn sheller and their thresher with a horse power."
2.NounAlternative spelling of horsepower (“unit of power equaling 745 watts or similar figure”).
3.NounUsed other than figuratively or idiomatically: motive effort supplied by one or more horses.
"In the 1920s through 1940s, the push for power farming aimed to replace horse power with tractor power on the farm."