Plain English Meaning
The powerful blast of air a helicopter pushes down beneath itself when its rotor is turning. It hits the ground and spreads outward, and it can be strong enough to rock parked airplanes or blow loose items around.
Full Definition
The strong column of air pushed downward and outward by a helicopter's main rotor as it generates lift. This air mass moves at high velocity and spreads outward along the ground, capable of disturbing or damaging nearby aircraft, equipment, and loose objects.
Context Anchor
Encountered during ground operations when taxiing, parking, or walking near helicopters or other aircraft with large rotating blades.
Derivation
Rotor refers to the rotating blade system of a helicopter. Downwash combines down with wash, where wash has long been used in to disturbed or displaced air behind by a moving (as in or wash). Together the term names the downward-moving of air a rotor produces.