In the road car context, a diffuser is a shaped section of the vehicle’s underbody (closer to the rear) that is similar to that of a Formula One racecar. Said section enhances the transition between the high-velocity airflow underneath the car and the much slower external airflow behind the car.
This enhancement is done by providing space for the airflow underneath the car to expand and decelerate. It also helps to fill, to a certain extent, the wake, which is the turbulent area of low pressure that is caused as the car moves through the air. The low pressure in question can cause drag.
Integrating the exhaust system/tailpipes with the diffuser area can also help extract the air from below the vehicle. The exhaust gases energise the boundary area, helping to raise the pressure of the fast-moving air at the exit of the diffuser.
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