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Downwash is the air that is deflected by flowing around an airfoil. This airfoil can be the wing of a plane, the rotor of a helicoptor or a propeller on a plane. There is a misconception that downwash is why wings create lift, saying that according to Newton's 3rd law, for every action, there is an equal and opposite reaction - therefore, if air is being pushed down off the wing or rotor, the air must be pushing back on the wing or rotor, creating lift. This isn't true, though, because downwash is a byproduct of lift, not the cause of lift.