Horsepower is the power developed in the cylinder(s) of an engine. Some of this is absorbed by friction in various engine-parts. The power that remains - ie the power that can actually be used - is called 'brake horsepower. It was so-called because the level of power was measured by means of applying a brake to stop the engine-shaft from turning. After Watt and his steam engine developments, 1 HP became a conventional mechanical unit described as "550 foot pounds per second", which is actually about half as strong again as a real horse!