horsepower

horse +‎ power: the unit was originally defined as the amount of power that a horse could provide.

Both non-metric and metric units of power were derived from effectively identical measurements of the power a draught horse could sustain over several hours, with the difference in watts solely due to different rounding errors to express that power in round numbers in the original non-SI units (ft·lbf/min and kgf⋅m/s respectively).

• (General American) enPR: hôrsʹpou'ər, IPA^(key): /ˈhɔɹsˌpaʊɚ/
• (Received Pronunciation) enPR: hôsʹpou'ə, IPA^(key): /ˈhɔːsˌpaʊə/

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horsepower (countable and uncountable, plural horsepowers or horsepower)

1. (uncountable) Power derived from the motion of a horse.
   • 2003, Gavin Weightman, What the Industrial Revolution Did for Us, page 57:

     The wheel was to have been turned by horsepower, but it was adapted to be driven by a mill-wheel on the river Derwent […]

2. A nonmetric unit of power (symbol hp) with various definitions, for different applications. The most common of them is probably the mechanical horsepower, approximately equal to 745.7 watts.
   • 2012 March 22nd, David Blockley, Engineering: A Very Short Introduction (309), Oxford University Press, →ISBN, chapter 2: “The age of gravity – time for work”, page 20:

     In the past, before the widespread adoption of SI units, the work that engines were capable of doing was compared with the work that horses could do – hence the term ‘horsepower’. Various people came up with various equivalencies, but the modern agreed definition is that 1 horsepower is 746 joules per second or 746 watts.

3. A metric unit (symbol often PS from the German abbreviation), approximately equal to 735.5 watts.
4. (uncountable) Strength.

   political horsepower

• hp

• shorepower