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.
The drag equation is essentially a statement that, under certain conditions, the drag force on any object is approximately proportional to the square of its velocity through the fluid. The required conditions are that the Reynolds number of the flow around the object must be high enough to create a turbulent wake (larger velocities, larger objects, and lower viscosities make for larger Reynolds numbers), and that the object does not approach the speed of sound in the fluid. A Cd equal to 1 would be obtained in a case where all of the fluid approaching the object is brought to rest, building up stagnation pressure over the whole front surface. The top figure shows a flat plate with the fluid coming from the right and stopping at the plate. The graph to the left of it shows equal pressure across the surface. In a real flat plate the fluid must turn around the sides, and full stagnation pressure is found only at the center, dropping off toward the edges as in the lower figure and graph. The Cd of a real flat plate would be less than 1, except that there will be a negative pressure (relative to ambient) on the back surface. The overall Cd of a real square flat plate is often given as 1.17. Flow patterns and therefore Cd for some shapes can change with the Reynolds number and the roughness of the surfaces.