is the apparent power
of an engine
, as measured
at the output shaft
It is in contrast to the the engine's IHP (Indicated Horsepower) which is calculated by mathematical theory and is often far from reality, once frictional losses are added in.
BHP is a more useful measure of automobile power, since it usually equates closer to the power that actually gets to the car's wheels, and is usually actually measured, versus calculated.
BHP appears to get its name from one of the methods used to measure it. You measure how much braking force is needed to stop the engine.
In reality, the measurement is usually an engine attached to dynamometer. The distinction between different TYPES of BHP (Argh!) are based on what accessories are left on the engine -- like oil and water pumps, alternator and radiator fan. Then, some are corrected for STP values of temperature, humidity and pressure.
If you are serious, pick a standards body -- probably either DIN or SAE -- and read their specs.