diffuser
Automotive engineering component creating downforce
A piece of automotive bodywork that is meant to deliver downforce by channeling air under the car, especially under the midsection and rear, so that it creates a low-pressure system under the car which sucks the vehicle to the ground. This increases traction and stability at high speeds. These are especially prevalent on high-performance sports cars, supercars and most racing cars when putting spoilers that create similar downforce would just cause too much drag.
If you don't see a big tailwing on the back of a modern supercar, look at the tail for a diffuser. The double-deck diffusers used by a few teams caused a scandal in Formula One over the 2009 season.