In aeronautics downwash is the air forced down by the aerodynamic action of a wing

Wing

A wing is an appendage with a surface that produces lift for flight or propulsion through the atmosphere, or through another gaseous or liquid fluid...

 or helicopter rotor blade in motion, as part of the process of producing lift

Lift (force)

A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...

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An aircraft produces aerodynamic lift by deflecting air downwards as downwash. This generates an equal and opposite upwards force on the wing called lift. When the downwash force exceeds the weight of the aircraft, the aircraft will rise.