Power, in engineering, is the rate at which work gets done rather than the amount of force involved. Horsepower is one of the units that rate can be quoted in. Georgia State University's HyperPhysics reference puts the definition plainly: "A horsepower is equal to 550 ft lb/s". In watts, NIST gives that horsepower as 7.456 999 E+02, so 745.7 watts to one decimal. That is the whole unit. Everything else about it is bookkeeping.
The bookkeeping is where the confusion starts. The same engine can appear with more than one power figure attached, because the number depends on where in the drivetrain it was measured and under what conditions. BHP is one such answer to "measured where", which is why it is a narrower term than plain horsepower rather than a different thing.
Power also is not the same measurement as torque, though the two are quoted side by side on every spec sheet and are related through engine speed. If the difference is the argument you keep having, torque against horsepower takes it apart properly.
Written by Craig Fearn, Petrol & Ink. Last updated .