A dimensionless number used in aerodynamics to describe the drag of a shape. This number is independent of the size of the object and is usually determined in a wind tunnel. It is part of the basic drag equation F=.5*rho*V^2*Cd*A where F is the drag force, rho is the air density, V is the air velocity and A is the cross sectional area. All of these, except Cd, are directly measurable in a wind tunnel so Cd can be thought of the "fudge factor" that accounts for all of the aerodynamic peculiarities of a shape. The Cd for most sport type hobby rockets is in the range of .4 to .5. See also Reynolds Number.