Ram air, a non-mechanical form of forced induction, is a feature on some vehicle bodies that lets outside air force its way into the air intake system. This is achieved by placing an opening somewhere on the body that allows air to flow in naturally as the vehicle moves forward. The faster the vehicle moves in relation to the air, the greater the overall effect.
The effects of ram air are similar to that of a turbo or supercharger, though it's not directly powered by the exhaust or engine, and the effect depends entirely on the vehicle's speed and the wind direction. A small number of today's vehicles come stock with this feature, usually American sports cars and European exotics. Ram air can be added to a vehicle that didn't come with it, but it usually involves a lot of custom design work.