Definition
The downward deflection of the airflow behind a wing as it produces lift. As the wing forces air downward, an equal and opposite reaction lifts the wing upward. Downwash also tilts the local airflow at the wing slightly downward, which reduces the effective angle of attack and contributes to induced drag.
Plain English
When a wing makes lift, it pushes a stream of air downward behind it. That downward stream is called downwash.
Context Anchor
Seen in discussions of lift, ground effect, takeoff, landing, and why an airplane may feel different when it is flying close to the runway.
Derivation
A plain compound of 'down' and 'wash' — 'wash' here means a flow or current of air or water left behind a moving object, the same sense as the 'wash' from a boat. So downwash literally means the downward flow of air left behind the wing.
Why Pilots Care
In ground effect the ground reduces downwash, lowering induced drag and improving climb performance on takeoff.
Grounding Statement
Picture the wing leaving a stream of air angled downward behind the airplane as it flies.
Intuition Check
Do not read “downwash” as water or cleaning. In this context, it means airflow directed downward by a wing producing lift.
Example Sentence 1
Close to the ground, the runway blocks part of the wing's downwash, which reduces induced drag and produces the lift gain known as ground effect.
Example Sentence 2
Each wing leaves a trail of downwash that rolls up into wingtip vortices.