A dimensionless number used in aerodynamics t' describe t' drag o' a shape. This number is independent o' t' size o' t' object and is usually determined in a wind tunnel. It is part o' t' basic drag equation F=.5*rho*V^2*Cd*A where F be t' drag force, rho be t' air density, V be t' air velocity and A be t' cross sectional area. All o' these, except Cd, me bucko, are directly measurable in a wind tunnel so Cd can be thought o' t' "fudge factor" that accounts for all o' t' aerodynamic peculiarities o' a shape. Blimey! T' Cd for most sport type hobby rockets is in t' range o' .4 t' .5. See also Reynolds Number.