Taken from -
http://en.wikipedia.org/wiki/Horsepower
"Brake horsepower (bhp) is the measure of an engine's horsepower without the loss in power caused by the gearbox, generator, differential, water pump, and other auxiliary components. Thus the prefix "brake" refers to where the power is measured: at the engine's output shaft, as on an engine dynamometer. The actual horsepower delivered to the driving wheels is less. An engine would have to be retested to obtain a rating in another system. The term "brake" refers to the original use of a band brake to measure torque during the test (which is multiplied by the engine RPM and a scaling constant to give horsepower)."