From AMS Glossary
A dimensionless ratio of the component of force parallel to the direction of flow (drag) exerted on a body by a fluid to the kinetic energy of the fluid multiplied by a characteristic surface area of the body.
In symbols, the drag coefficient CD is
where ρ is the fluid density, U the speed, and L a characteristic length of the body. Rayleigh's formula expresses the drag coefficient as proportional to a power of the Reynolds number (Re),
For Couette flow, n = -1 and the constant is 2. In most aerodynamic experiments n lies between -1/4 and -1/2. The drag coefficient for the force exerted by the atmosphere on the earth is called the skin-friction coefficient.