From AMS Glossary
(Or resistance.) The frictional impedance offered by air to the motion of bodies passing through it. More precisely, the component of aerodynamic force parallel to the direction of mean flow.
At very low speeds, most of the drag exerted by the air on a body moving through it is due to viscous drag (or skin friction) acting through a fairly thin boundary layer. In the case of spheres, the low-speed air resistance is given by Stokes's law. For higher speeds, so-called form drag or pressure drag arises as a result of separation of the laminar boundary layer creating a wake region of chaotic flow in which the pressure is reduced. In general, for large Reynolds numbers, form drag is far more significant than viscous drag. The velocity dependence of air resistance changes more or less continuously from linear dependence in the viscous range to velocity-squared dependence at high speeds. The latter dependence is given by Rayleigh's formula,
where ρ is the density of the medium, L a characteristic linear dimension of the body, and U the speed of the body relative to the fluid. The drag coefficient CD is proportional to an experimentally determined power of the Reynolds number. At speeds approaching that of sound, an additional source of drag arises out of the formation of shock waves.