Plain English Meaning
The fast, swirling stream of air thrown backward by the spinning propeller. It hits the back of the airplane and affects how the tail surfaces respond.
Full Definition
The disturbed, accelerated mass of air pushed rearward by a turning propeller. This airflow is faster, more turbulent, and rotates in a corkscrew pattern around the fuselage, striking the tail surfaces and other parts of the airframe behind the propeller.
Context Anchor
In an elevator trim stall discussion, propwash matters when power is added and the stronger airflow over the tail can make the nose rise more strongly.
Derivation
A simple compound of 'prop' (propeller) and 'wash' (a disturbed flow of or air behind by something moving through it, as in the of a boat). The term captures the idea of a , churned-up flow.