Power-to-weight ratio is a measure commonly used when comparing various vehicles (or engines), including automobiles, motorcycles and aircraft. It is, simply, the power the engine develops, divided by the vehicle's or (or engine) weight.
The power-to-weight ratio is often used as an indication of likely accelerative performance. Vehicle weights have relatively little impact on top speed, which is mostly dependent on aerodynamic drag (see drag equation). Acceleration, on the other hand, is dominated by the Newtonian acceleration term, 
, so more force (F - from the engine's torque delivered to the driven wheels or thrust delivered by an aircraft engine), will deliver more acceleration (a) for any given vehicle mass (m = weight/g).
In any vehicle the engine power-to-weight ratio is essential for vehicle power-to-weight ratio. But in an aircraft it's more critical than in any other vehicle because any additional weight requires more lift to be generated by the wings in order to lift it. More lift from the wings automatically means more drag, through a process known as induced drag, slowing the plane down. Thus if any two engines deliver the same power, the lighter one will result in a better plane. Power-to-weight ratio therefore has a much more important impact on overall performance in aircraft, including top speed.
In this usage the power-to-weight ratio is typically used to refer to the weight of the engine alone, as a useful way of comparing various aircraft engines. The term applying to the aircraft as a whole is power loading, and is used especially in helicopter engineering.
