From The Collaborative International Dictionary of English v.0.48 [gcide]:
3. (Aeronautics) A device attached to the wing of an airplane or other airfoil, which breaks the smooth flow of air during flight and decreases the lift of the airfoil; -- used to control the attitude of the airplane during banking or descent. [PJC]
4. A device attached to a car to decrease lift and increase traction, usually shaped as a flat surface and attached above the rear of the car, and working on the same principal as the aircraft spoiler. [PJC]
From WordNet (r) 3.0 (2006) [wn]:
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