The definition of Cd

As a first approximation the drag force, (friction force due to the generation of turbulence), on golf balls, and round stones in water, is given by ...

... where r is the density of the medium, A is the cross sectional area, and v is the velocity. The drag coefficient is defined as the measured drag, divided by the density of the medium, times the frontal area, times one half the velocity squared.

The drag coefficient for a particular body depends on the shape, the surface texture, and the orientation.

Measuring Cd

When a body is falling at terminal velocity, the drag force is given by the weight, minus the buoyancy force. The buoyancy force, equal to the weight of fluid displaced, is small when the medium is air, and is often neglected.