The horsepower (hp) 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 existence of an SI unit for power, the watt (W). However, the idea of horsepower persists as a legacy term in many languages, particularly in the automotive industry for listing the maximum power output of internal-combustion engines.
These factors can be combined in unexpected ways — the true power output for an engine rated at "100 horsepower" might vary from a reader's expectations. For this reason, various groups have attempted to standardize both the definition and the measurement system, often leading to more confusion. By comparison, the watt, defined by the International System of Units ("SI"), is not subject to varying definitions.