Plain English Meaning

When a wing makes lift, it pushes air downward. The air leaving the back of the wing is now moving slightly downward instead of straight back. That downward-moving air behind the wing is the downwash.

Full Definition

The downward deflection of airflow behind a wing as it produces lift. As the wing pushes air down, that mass of air leaves the trailing edge angled downward relative to the free airstream, and this downward-angled flow is called downwash.

Context Anchor

Seen in discussions of flaps, lift, pitch changes, and how airflow from the wing affects the tail of the airplane.

Derivation

‘Downwash’ is a plain compound: ‘down’ plus ‘wash,’ where ‘wash’ here means a stream or of moving air or (as in ‘the wash from a boat’). So it literally means the downward stream of air behind by the wing.