Plain English Meaning
As a wing moves forward, it pulls some of the air ahead of it slightly upward before that air actually reaches the wing. That upward motion of the air is called upwash.
Full Definition
The upward deflection of air ahead of and around a wing as it moves through the air, caused by the wing's circulation pattern. Upwash occurs in front of the wing and along its outer edges, where air is drawn upward before flowing over and around the wing.
Context Anchor
Seen in discussions of ground effect during takeoff, where the runway changes the airflow pattern around the wing close to the ground.
Derivation
A simple compound: 'up' (the direction) plus 'wash,' which in aerodynamics means a flow of air pushed by a wing or . So upwash literally means air being washed upward by the wing's influence.