Sentence:  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 ‘<WSD>horsepower</WSD>'. Various people came up with various equivalencies, but the modern agreed definition is that 1 <WSD>horsepower</WSD> is 746 joules per second or 746 watts. 
Meaning of the word marked with <WSD> tags:  A non-metric 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.