What is horsepower in bikes?

Horsepower (hp or HP[1]) is the name of several non-metric units of power. In scientific discourse, the term "horsepower" is rarely used because of its various definitions and the already existent SI unit for power, the watt (W). However, use of the term "horsepower" persists as a legacy in many languages and industries, particularly as a units of measurement of the maximum power output of internal-combustion engines of automobiles, and often of trucks, buses and ships. The use of 'HP' is being slowly replaced by kW (kilowatt) and MW (megawatt).