Spoiler

A spoiler is an aerodynamic device that is designed to 'spoil' the movement of air (often called drag) across a vehicle body while it is moving. 

Spoilers are often fitted to high-performance sporty cars, where they can help make the car more stable at high speeds. Many spoilers on more mainstream cars are purely for style, though, and have little aerodynamic benefit. Sometimes, they even make a car less aerodynamic.