Plain English Meaning

When a wing makes lift, it pushes a stream of air downward behind it. That downward stream is called downwash.

Full 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.

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 of 'down' and 'wash' — 'wash' here means a flow or of air or 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.