Drag coefficient

number that represents the intensity of force acting on a body in motion in direct opposition to the body's motion

The drag coefficient (Cd, Cx or Cw, depending on the country) is a dimensionless quantity that describes a characteristic amount of aerodynamic drag caused by fluid flow, used in the drag equation. Two objects of the same frontal area moving at the same speed through a fluid will experience a drag force proportional to their Cd numbers. Coefficients for rough unstreamlined objects can be 1 or more, for smooth objects much less.
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Noun
1. the ratio of the drag on a body moving through air to the product of the velocity and the surface area of the body
(synonym) coefficient of drag
(hypernym) coefficient