The area between the flat surface of a cylinder head and the top flat surface of a piston at top dead center, usually the thickness of the head gasket. Also called the 'pinch' or 'squish' area. Cylinder head chambers that extend to the full circular width of the cylinder bore will not have a 'quench' area, neither will a motor with 'dished' or a recessed top in the pistons.
In a motor with a true quench area, the air is violently 'pinched' out from between the rising piston and the head surface causing a desirable swirling effect. This will better mix, or homogenize the fuel and air, producing a more efficient burn. This in turn results in an even distribution of heat produced, reducing hot spots which lead to pre-ignition.